release GIL for computationally expensive C calls #172
Description
Thank you for maintaining this library, I am considering using it in my https://github.com/fommil/luddcam capture app to find star centers for guiding and to avoid having to pass fits files back and forth to astrometry's solve-field (by writing out .axy files).
My app runs realtime on low powered (raspberry pi 4b) devices and it has quite a few threads in play to do various things.
Unfortunately it looks like the calls to the sep C library are not flagged to release the Global Interpreter Lock (GIL) so while the C code is doing its thing, the rest of the python code is blocked from progressing.
Is there any chance you could please do the relevant save/restore thread dance so that it becomes a non-blocking api? You're read only of the input array so this should be pretty safe. There's of course the a risk that the user mutates the data array in another thread, but that sounds like it was always asking for trouble.
Note that I made a similar request of fitsio over at esheldon/fitsio#476 and their WIP implementation is a really excellent tutorial on how to release the GIL from native library code.