handling metal-poor atmospheres with negative optical depths more gracefully

Hi,

I don't know if it's worth reporting, but I looked at korg failures over large grid that are not 'chemistry convergence' issues.
And this
thing stand out:

```
ERROR: LoadError: DomainError with -7.986061283680538e-6:
log was called with a negative real argument but will only return a complex result if called with a complex argument. Try log(Complex(x)).
Stacktrace:
[1] throw_complex_domainerror(f::Symbol, x::Float64)
@ Base.Math ./math.jl:33
[2] _log(x::Float64, base::Val{:ℯ}, func::Symbol)
@ Base.Math ./special/log.jl:301
[3] log
@ ./special/log.jl:267 [inlined]
[4] _broadcast_getindex_evalf
@ ./broadcast.jl:709 [inlined]
[5] _broadcast_getindex
@ ./broadcast.jl:682 [inlined]
[6] getindex
@ ./broadcast.jl:636 [inlined]
[7] macro expansion
@ ./broadcast.jl:1004 [inlined]
[8] macro expansion
@ ./simdloop.jl:77 [inlined]
[9] copyto!
@ ./broadcast.jl:1003 [inlined]
[10] copyto!
@ ./broadcast.jl:956 [inlined]
[11] copy
@ ./broadcast.jl:928 [inlined]
[12] materialize
@ ./broadcast.jl:903 [inlined]
[13] radiative_transfer(α::Matrix{Float64}, S::Matrix{Float64}, spatial_coord::Vector{Float64}, μ_points::Int64, spherical::Bool; include_inward_rays::Bool, α_ref::Vector{Float64}, τ_ref::Vector{Float64}, I_scheme::String, τ_scheme::String)
@ Korg.RadiativeTransfer ~/.julia/packages/Korg/6TfTZ/src/RadiativeTransfer/RadiativeTransfer.jl:110
[14] radiative_transfer
@ ~/.julia/packages/Korg/6TfTZ/src/RadiativeTransfer/RadiativeTransfer.jl:73 [inlined]
[15] #radiative_transfer#1
@ ~/.julia/packages/Korg/6TfTZ/src/RadiativeTransfer/RadiativeTransfer.jl:61 [inlined]
[16] synthesize(::Korg.PlanarAtmosphere{Float64, Float64, Float64, Float64, Float64}, ::Vector{Korg.Line{Float64, Float64, Float64, Float64, Float64, Float64}}, ::Vector{Float64}, ::Int64, ::Vararg{Int64}; vmic::Int64, line_buffer::Int64, cntm_step::Float64, air_wavelengths::Bool, hydrogen_lines::Bool, use_MHD_for_hydrogen_lines::Bool, hydrogen_line_window_size::Int64, mu_values::Int64, line_cutoff_threshold::Float64, electron_number_density_warn_threshold::Float64, electron_number_density_warn_min_value::Float64, return_cntm::Bool, I_scheme::String, tau_scheme::String, ionization_energies::Dict{UInt8, Vector{Float64}}, partition_funcs::Dict{Korg.Species, Korg.CubicSplines.CubicSpline{Vector{Float64}, Vector{Float64}, Vector{Float64}, Vector{Float64}, Float64}}, log_equilibrium_constants::Dict{Korg.Species, Union{Korg.CubicSplines.CubicSpline{Vector{Float64}, Vector{Float64}, Vector{Float64}, Vector{Float64}, Float64}, Korg.var"#logK#57"{Korg.Species, Dict{Korg.Species, Korg.CubicSplines.CubicSpline{Vector{Float64}, Vector{Float64}, Vector{Float64}, Vector{Float64}, Float64}}}}}, molecular_cross_sections::Vector{Any}, use_chemical_equilibrium_from::Nothing, verbose::Bool)
@ Korg ~/.julia/packages/Korg/6TfTZ/src/synthesize.jl:224
[17] top-level scope

```
This is caused by this log
https://github.com/ajwheeler/Korg.jl/blob/7516c46281f110742749a1f3e3aad56c5c337306/src/RadiativeTransfer/RadiativeTransfer.jl#L110

The reproducer (requires the linelist that i previously shared)

```
using Korg
using Serialization

lines=open(deserialize, "vald_all_3000_10000.julia.bin")
teff=(4118.689742870629)
logg=(5.291771519929171)
X_H=[12. , 10.93 , -2.84274731, -2.51274731, -1.19274731,
4.49725269, 3.88725269, 5.35591968, 0.66725269, 4.53591968,
2.27725269, 4.22591968, 2.47725269, 4.20591968, 1.46725269,
3.83591968, 1.60725269, 2.87591968, 1.18725269, 3.00591968,
-0.72274731, 1.59591968, 0.10725269, 1.74725269, 1.49725269,
3.55725269, 1.02725269, 2.33725269, 0.31725269, 0.70725269,
-1.01274731, -0.31274731, -1.60274731, -0.56274731, -1.33274731,
-0.64274731, -1.29274731, -0.97274731, -1.68274731, -1.31274731,
-2.47274731, -1.97274731, -8.89274731, -2.05274731, -2.77274731,
-2.23274731, -2.95274731, -2.12274731, -2.29274731, -1.89274731,
-2.89274731, -1.70274731, -2.38274731, -1.65274731, -2.82274731,
-1.72274731, -2.76274731, -2.19274731, -3.31274731, -2.44274731,
-8.89274731, -2.89274731, -3.37274731, -2.78274731, -3.61274731,
-2.75274731, -3.38274731, -2.96274731, -3.89274731, -2.81274731,
-3.83274731, -3.01274731, -4.06274731, -2.78274731, -3.66274731,
-2.64274731, -2.51274731, -2.25274731, -2.88274731, -2.76274731,
-2.99274731, -1.89274731, -3.24274731, -8.89274731, -8.89274731,
-8.89274731, -8.89274731, -8.89274731, -8.89274731, -3.83274731,
-8.89274731, -4.41274731]
marcs = Korg.interpolate_marcs((teff),
(logg),
X_H,
clamp_abundances=true)

minlam=3400
maxlam=5000
spec = Korg.synthesize(marcs,
lines,X_H,minlam,maxlam,vmic=1,
line_buffer=100,
hydrogen_line_window_size=400)
```