Consider adding fallback median implementation

[mwcraig]

The suggestion was made to include an array API-compliant median function and include it in ccdproc. The comment and a potential implementation are in a comment on #885.

Some questions to answer:

1. Do all of the array libraries have (at a minimum) median along an axis?
2. Do all of them have some variant on nanmedian?
3. How does performance of this compare to converting to numpy and using that (ignoring GPU/CPU stuff for the moment)?