Add Fermi-Dirac-Integral

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.

Start by reading the two linked Fermi–Dirac integral references and determine the intended definition, normalization, and approximation. No source file, test, or entry point is named, so the API, implementation location, and completion criteria still need to be established.

Written by the indexing model from the issue text.