Add RELXILL_NK

[fjebaker]

• https://www.tat.physik.uni-tuebingen.de/~nampalliwar/relxill_nk/

[phajy]

The routine I'm most interested in to start with is relline_nk to allow direct comparisons with Gradus.jl line profiles.

[phajy]

While the code exists, there is no public table of transfer functions.

Update 2: the link at the top no longer works.