If all buses within a zone have a Pd of zero, model building will crash

# :beetle:

- [x] I have checked that this issue has not already been reported.

### Bug summary
If all buses within a zone have a `Pd` of zero, model building will crash within the `_add_constraint_power_balance!` function, due to a `NaN` value within `bus_demand`. The origin of this is `NaN` value is the following line: `bus_share = bus_df[:, :load] ./ bus_df_with_zone_load[:, :load_sum]` which divides by zero for at least one zone since the sum of all Pd values for the zone is zero.

https://github.com/Breakthrough-Energy/REISE.jl/blob/420dc059a785dc3db3781734ec0e264f31f04a28/src/model.jl#L55

### Code for reproduction
Can be reproduced using the current HIFLD grid model for the Eastern interconnect, which has all zeros for `Pd` values for buses within the `"Colorado Eastern"` zone (zone_id = 6). To reproduce, you can call `REISE.run_scenario_gurobi` or `REISE.run_scenario`. Checking the data with python we see the root of the data problem.
```python
>>> from powersimdata import Grid
>>> eastern = Grid("Eastern", "hifld")
>>> eastern.bus.groupby("zone_id")["Pd"].sum()
zone_id
1 10011.178932
2 6151.994940
6 0.000000
7 7185.898740
8 1924.068480
9 42205.325171
10 21388.176215
11 6307.999616
13 25755.139020
14 13525.860755
15 5916.248707
16 9188.076334
17 9426.187824
18 13712.366730
19 12097.884480
20 2684.338920
21 20058.991980
22 11412.078189
23 12401.671927
24 6132.018522
25 107.257620
27 20757.000844
28 1584.745305
29 3870.868983
31 2709.729240
32 17958.609315
33 555.734850
36 39349.735830
37 23618.239177
38 7985.510489
40 25689.231120
41 2125.034310
42 10309.792483
43 1495.255011
45 13807.662019
46 5488.202250
50 17033.393106
51 1254.869130
53 11788.683081
54 3645.733980
Name: Pd, dtype: float64
```

### Actual outcome
The output produced by the above code, which may be a screenshot, console output, etc.
```julia
Expression contains an invalid NaN constant. This could be produced by `Inf - Inf`.

Stacktrace:
[1] error(::String) at .\error.jl:33
[2] _assert_isfinite(::JuMP.GenericAffExpr{Float64,JuMP.VariableRef}) at C:\Users\DanielOlsen\.julia\packages\JuMP\Xrr7O\src\aff_expr.jl:476
[3] MathOptInterface.ScalarAffineFunction(::JuMP.GenericAffExpr{Float64,JuMP.VariableRef}) at C:\Users\DanielOlsen\.julia\packages\JuMP\Xrr7O\src\aff_expr.jl:507
[4] moi_function at C:\Users\DanielOlsen\.julia\packages\JuMP\Xrr7O\src\aff_expr.jl:550 [inlined]
[5] moi_function at C:\Users\DanielOlsen\.julia\packages\JuMP\Xrr7O\src\constraints.jl:396 [inlined]
[6] add_constraint(::JuMP.Model, ::JuMP.ScalarConstraint{JuMP.GenericAffExpr{Float64,JuMP.VariableRef},MathOptInterface.EqualTo{Float64}}, ::String) at C:\Users\DanielOlsen\.julia\packages\JuMP\Xrr7O\src\constraints.jl:547
[7] add_constraint at C:\Users\DanielOlsen\.julia\packages\JuMP\Xrr7O\src\constraints.jl:546 [inlined]
[8] _broadcast_getindex_evalf at .\broadcast.jl:648 [inlined]
[9] _broadcast_getindex at .\broadcast.jl:621 [inlined]
[10] getindex at .\broadcast.jl:575 [inlined]
[11] copyto_nonleaf!(::Array{JuMP.ConstraintRef{JuMP.Model,MathOptInterface.ConstraintIndex{MathOptInterface.ScalarAffineFunction{Float64},MathOptInterface.EqualTo{Float64}},JuMP.ScalarShape},2}, ::Base.Broadcast.Broadcasted{Base.Broadcast.DefaultArrayStyle{2},Tuple{Base.OneTo{Int64},Base.OneTo{Int64}},typeof(JuMP.add_constraint),Tuple{Base.RefValue{JuMP.Model},Base.Broadcast.Broadcasted{Base.Broadcast.DefaultArrayStyle{2},Nothing,typeof(JuMP.build_constraint),Tuple{Base.RefValue{JuMP.var"#_error#92"{Tuple{Symbol,Symbol,Expr},Symbol,LineNumberNode}},Base.Broadcast.Extruded{Array{JuMP.GenericAffExpr{Float64,JuMP.VariableRef},2},Tuple{Bool,Bool},Tuple{Int64,Int64}},Base.RefValue{MathOptInterface.EqualTo{Float64}}}}}}, ::CartesianIndices{2,Tuple{Base.OneTo{Int64},Base.OneTo{Int64}}}, ::CartesianIndex{2}, ::Int64) at .\broadcast.jl:1026
[12] copy at .\broadcast.jl:880 [inlined]
[13] materialize(::Base.Broadcast.Broadcasted{Base.Broadcast.DefaultArrayStyle{2},Nothing,typeof(JuMP.add_constraint),Tuple{Base.RefValue{JuMP.Model},Base.Broadcast.Broadcasted{Base.Broadcast.DefaultArrayStyle{2},Nothing,typeof(JuMP.build_constraint),Tuple{Base.RefValue{JuMP.var"#_error#92"{Tuple{Symbol,Symbol,Expr},Symbol,LineNumberNode}},Array{JuMP.GenericAffExpr{Float64,JuMP.VariableRef},2},Base.RefValue{MathOptInterface.EqualTo{Float64}}}}}}) at .\broadcast.jl:837
[14] macro expansion at C:\Users\DanielOlsen\.julia\packages\JuMP\Xrr7O\src\macros.jl:677 [inlined]
[15] _add_constraint_power_balance!(::JuMP.Model, ::REISE.Case, ::REISE.Sets, ::REISE.Storage, ::REISE.DemandFlexibility, ::Array{Float64,2}, ::Bool) at C:\Users\DanielOlsen\repos\bes\REISE.jl\src\model.jl:278
[16] _build_model(::JuMP.Model; case::REISE.Case, storage::REISE.Storage, demand_flexibility::REISE.DemandFlexibility, start_index::Int64, interval_length::Int64, demand_scaling::Float64, load_shed_enabled::Bool, load_shed_penalty::Int64, trans_viol_enabled::Bool, trans_viol_penalty::Int64, initial_ramp_enabled::Bool, initial_ramp_g0::Array{Float64,1}, storage_e0::Array{Float64,1}, init_shifted_demand::Array{Float64,1}) at C:\Users\DanielOlsen\repos\bes\REISE.jl\src\model.jl:728
[17] interval_loop(::Gurobi.Env, ::Dict{String,Any}, ::Dict{String,Int64}, ::Int64, ::Int64, ::Int64, ::String, ::String) at C:\Users\DanielOlsen\repos\bes\REISE.jl\src\loop.jl:73
[18] (::REISE.var"#112#113"{Int64,Int64,Int64,String,Gurobi.Env})() at C:\Users\DanielOlsen\repos\bes\REISE.jl\src\REISE.jl:91
[19] #100 at C:\Users\DanielOlsen\repos\bes\REISE.jl\src\save.jl:120 [inlined]
[20] redirect_stderr(::REISE.var"#100#104"{REISE.var"#112#113"{Int64,Int64,Int64,String,Gurobi.Env}}, ::IOStream) at .\stream.jl:1150
[21] #99 at C:\Users\DanielOlsen\repos\bes\REISE.jl\src\save.jl:119 [inlined]
[22] redirect_stdout(::REISE.var"#99#103"{IOStream,REISE.var"#112#113"{Int64,Int64,Int64,String,Gurobi.Env}}, ::IOStream) at .\stream.jl:1150
[23] #98 at C:\Users\DanielOlsen\repos\bes\REISE.jl\src\save.jl:118 [inlined]
[24] open(::REISE.var"#98#102"{IOStream,REISE.var"#112#113"{Int64,Int64,Int64,String,Gurobi.Env}}, ::String, ::Vararg{String,N} where N; kwargs::Base.Iterators.Pairs{Union{},Union{},Tuple{},NamedTuple{(),Tuple{}}}) at .\io.jl:325
[25] open at .\io.jl:323 [inlined]
[26] #97 at C:\Users\DanielOlsen\repos\bes\REISE.jl\src\save.jl:117 [inlined]
[27] open(::REISE.var"#97#101"{REISE.var"#112#113"{Int64,Int64,Int64,String,Gurobi.Env},String}, ::String, ::Vararg{String,N} where N; kwargs::Base.Iterators.Pairs{Union{},Union{},Tuple{},NamedTuple{(),Tuple{}}}) at .\io.jl:325
[28] open at .\io.jl:323 [inlined]
[29] redirect_stdout_stderr(::REISE.var"#112#113"{Int64,Int64,Int64,String,Gurobi.Env}, ::String, ::String) at C:\Users\DanielOlsen\repos\bes\REISE.jl\src\save.jl:116
[30] run_scenario(; num_segments::Int64, interval::Int64, n_interval::Int64, start_index::Int64, inputfolder::String, outputfolder::String, threads::Nothing, optimizer_factory::Gurobi.Env, solver_kwargs::Dict{String,Int64}, model_kwargs::Dict{String,Integer}) at C:\Users\DanielOlsen\repos\bes\REISE.jl\src\REISE.jl:89
[31] run_scenario_gurobi(; solver_kwargs::Nothing, kwargs::Base.Iterators.Pairs{Symbol,Any,NTuple{6,Symbol},NamedTuple{(:interval, :n_interval, :start_index, :inputfolder, :outputfolder, :model_kwargs),Tuple{Int64,Int64,Int64,String,String,Dict{String,Integer}}}}) at C:\Users\DanielOlsen\repos\bes\REISE.jl\src\solver_specific\gurobi.jl:13
```

### Expected outcome
In the case I was running into, I had set the demand to zero for that zone anyway, so I would have hoped that `bus_demand` would be all zeros for that zone. In this situation, I think we can safely replace any NaN values with zeros, and potentially warn the user/log of what's going on. In a situation for which there is non-zero demand for a zone with all-zero Pd values, I think we should detect this when loading the input files and halt before trying to build the model.

### Environment
This bug should be environment agnostic.

Please specify your platform and versions of the relevant libraries you are using:
* Operating system: Windows 10
* REISE.jl revision (run `git rev-parse origin/HEAD`): `e8639dd8a2fe42299e58e0c7ed49bc84527f5d07`
* Julia version: 1.5.4