Add Fermi-Dirac-Integral

[feanor12]

Would this be the place where one can add the Fermi-Dirac-Integral?

https://de.wikipedia.org/wiki/Fermi-Dirac-Integral (contains approximation)
https://en.wikipedia.org/wiki/Complete_Fermi%E2%80%93Dirac_integral

It is a quite useful function in solid state physics.

[feanor12]

I created a small package here: FermiDiracIntegrals.jl

Additional approximations might be interesting for time-sensitive calculations.

[MartinMikkelsen]

For future reference, I have implemented the method from this article. This contains Fermi-Dirac integrals of orders -1/2, 1/2, 3/2, and 5/2 over the entire range to high precision.

[stevengj]

I think in the future we are planning to split SpecialFunctions into multiple subpackages for different special functions, rather than adding more special functions here.