Fix (or at least explain and hide) astropy WCS warnings for MOST tutorial

[bsipocz]

There are quite a lot of astropy warnings rendered into the docs (40 at the time of writing), ideally those should be cleaned up either by changing a code or filtered out with an explanation of why they cannot be fixed.

https://caltech-ipac.github.io/irsa-tutorials/most-queries/

E.g.

stack = plot_cutouts(most_output,return_stack=True)

WARNING: FITSFixedWarning: RADECSYS= 'FK5 ' 
the RADECSYS keyword is deprecated, use RADESYSa. [astropy.wcs.wcs]

WARNING: FITSFixedWarning: RADECSYS= 'FK5 ' 
the RADECSYS keyword is deprecated, use RADESYSa. [astropy.wcs.wcs]

WARNING: FITSFixedWarning: RADECSYS= 'FK5 ' 
the RADECSYS keyword is deprecated, use RADESYSa. [astropy.wcs.wcs]
...