Implement special functions

[certik]

There are lots of implementations online.

As an example, here are the special functions that I needed in my projects over the past 10 years:

https://github.com/certik/fortran-utils/blob/b43bd24cd421509a5bc6d3b9c3eeae8ce856ed88/src/special.f90

https://github.com/certik/hfsolver/blob/b4c50c1979fb7e468b1852b144ba756f5a51788d/src/special_functions.f90

Being able to standardize on the API for all or most of them would be a huge deal. Other languages:

SciPy

https://docs.scipy.org/doc/scipy/reference/special.html

Matlab

https://www.mathworks.com/help/matlab/special-functions-1.html

Julia

Separate package: SpecialFunctions.jl

[14NGiestas]

Closing it in favor of #305