Make SEP optional

[juliotux]

SEP is not being maintened and do not support newer Python and Numpy versions. There are forks pushing updates, like sep-pjw and sdss-sep, but with different names. This can break some environments.

Make sep an optional dependency with import checking when needed, or a conditional dependency as defined by PEP 631 may solve the problem.

[martinberoiz]

SEP is not being maintened and do not support newer Python and Numpy versions

That's sad to hear.

I need to investigate this more. If I make it an optional dependency, I need to be able to replace it with another similar package because source extraction is core to the library.