bug: overly permissive methods causing ambiguity

Some of the methods defined in this package, such as the two-argument methods for the arithmetic functions, such as `+`, `-`, `*`, `/` have type signatures that are invalid because of being overly permissive. The issue is that ambiguity arises as soon as another package decides to do the same thing. Example:

```julia-repl
julia> using DynamicQuantities

julia> q = 0.2u"m/s"
0.2 m s⁻¹

julia> struct Z <: Number end

julia> Base.:(*)(::Number, z::Z) = z

julia> q * Z()
ERROR: MethodError: *(::Quantity{Float64, Dimensions{FixedRational{Int32, 25200}}}, ::Z) is ambiguous.

Candidates:
*(l::AbstractQuantity, r::Number)
@ DynamicQuantities ~/tmp/jl/DynamicQuantities.jl/src/math.jl:19
*(::Number, z::Z)
@ Main REPL[4]:1

Possible fix, define
*(::AbstractQuantity, ::Z)

Stacktrace:
[1] top-level scope
@ REPL[5]:1
```

The solution is to set a more strict constraint for the "other type" in the type signature. `Number` is too permissive, as (some of) the quantity types defined here also subtype `Number`.

Some of the arithmetic methods defined in this package, those defined for `AbstractGenericQuantity`, do not constrain the "other type" at all!