Enhance promotion of SimpleRatio to work with FixedPointNumbers

[mkitti]

Could we enhance the promotion of SimpleRatio so that it can work with FixedPointNumbers? Many images are encoded in FixedPointNumbers and this would be useful to apply Interpolations to them directly.

Minimum Working Example:

julia> using Ratios, FixedPointNumbers

julia> SimpleRatio(5,3) * 0.035N0f8
ERROR: promotion of types SimpleRatio{Int64} and N0f8 failed to change any arguments
Stacktrace:
 [1] error(::String, ::String, ::String)
   @ Base .\error.jl:42
 [2] sametype_error(input::Tuple{SimpleRatio{Int64}, N0f8})
   @ Base .\promotion.jl:316
 [3] not_sametype(x::Tuple{SimpleRatio{Int64}, N0f8}, y::Tuple{SimpleRatio{Int64}, N0f8})
   @ Base .\promotion.jl:310
 [4] promote
   @ .\promotion.jl:293 [inlined]
 [5] *(x::SimpleRatio{Int64}, y::N0f8)
   @ Base .\promotion.jl:322
 [6] top-level scope
   @ REPL[51]:1

Motivating Example:
xref: JuliaMath/Interpolations.jl#413
cc: @johnnychen94

julia> using FixedPointNumbers, Interpolations

julia> r = reinterpret(N0f8,[0x73, 0x94, 0x66, 0x1e])
4-element reinterpret(N0f8, ::Vector{UInt8}):
 0.451N0f8
 0.58N0f8
 0.4N0f8
 0.118N0f8

julia> itp = interpolate(r, BSpline(Quadratic(Flat(OnGrid()))));

julia> itp
4-element interpolate(OffsetArray(::Array{N0f8,1}, 0:5), BSpline(Quadratic(Flat(OnGrid())))) with element type Float64:
Error showing value of type Interpolations.BSplineInterpolation{Float64, 1, OffsetArrays.OffsetVector{N0f8, Vector{N0f8}}, BSpline{Quadratic{Flat{OnGrid}}}, Tuple{Base.OneTo{Int64}}}:
ERROR: promotion of types Ratios.SimpleRatio{Int64} and N0f8 failed to change any arguments
Stacktrace:
  [1] error(::String, ::String, ::String)
    @ Base .\error.jl:42
  [2] sametype_error(input::Tuple{Ratios.SimpleRatio{Int64}, N0f8})
    @ Base .\promotion.jl:316
  [3] not_sametype(x::Tuple{Ratios.SimpleRatio{Int64}, N0f8}, y::Tuple{Ratios.SimpleRatio{Int64}, N0f8})
    @ Base .\promotion.jl:310
  [4] promote
    @ .\promotion.jl:293 [inlined]
  [5] *(x::Ratios.SimpleRatio{Int64}, y::N0f8)
    @ Base .\promotion.jl:322

[mkitti]

@timholy any thoughts on the best way to proceed with this?

[timholy]

I'm surprised this hasn't come up before! See #19 (feel free to review and let me know what you don't like).