Change fallback unsafe_gamma for x::Real to x<:AbstractFloat. Add

[cgeoga]

This PR tweaks the behavior of unsafe_gamma to avoid stack overflow errors with ForwardDiff.jl. The method unsafe_gamma(x::Real) = unsafe_gamma(float(x)) has been given different type bounds as that particular dispatch method seems prone to these issues (ref JuliaLang/julia#26552). Further adds a generic fallback to gamma from SpecialFunctions.jl.

This design decision may not be coherent with other choices here, like manually implementing unsafe_gamma(x::Dual), so please let me know if this is not acceptable. There is some discussion available on the issue here. This method avoids needing to make ForwardDiff or DiffRules a new dependency.

[codecov]

Codecov Report

Merging #28 into master will decrease coverage by 2.35%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master      #28      +/-   ##
==========================================
- Coverage   79.88%   77.53%   -2.36%     
==========================================
  Files           6        6              
  Lines         900      899       -1     
==========================================
- Hits          719      697      -22     
- Misses        181      202      +21     

Impacted Files	Coverage Δ
src/specialfunctions.jl	53.99% <0.00%> (-6.52%)	⬇️
src/gauss.jl	53.00% <0.00%> (-1.00%)	⬇️

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[MikaelSlevinsky]

Hey thanks!

If you could bump the version when you're happy, then I could register a new release upon merging.

[cgeoga]

Thanks so much! I just pushed up a new tag. That is what it means to bump the version, right? Sorry---first time doing this.

[MikaelSlevinsky]

I mean change this line

HypergeometricFunctions.jl/Project.toml

Line 3 in 720e95d

version = "0.3.1"

to version = "v0.3.2".

Then, I can use the @JuliaRegistrator GitHub bot to register the new version in the General registry (and then a tag/release will also be made to this repository).

[cgeoga]

Oof, apologies. Just pushed up a commit that does that.

Also, I played with the dispatch one last time, removing unsafe_gamma(z<:AbstractFloat) = unsafe_gamma(z) and unsafe_gamma(z<:Complex) = gamma(z), as those should both be covered by the fallback unsafe_gamma(z) = gamma(z). Assuming that is agreeable, I think that should do it.

EDIT: Very sorry for so many tiny commits. I could have sworn I removed the commented line before, but I didn't. I fixed that in yet another commit.