MAINT: astroquery.mast caches into root cache

[bsipocz]

I've noticed that a lot of caching directories are created for astroquery.mast, mostly due to the fact that there are multiple classes provided in the module (as opposed to other modules that more strictly following the original astroquery API). I think it would be best to consolidate them into a mast base directory as Observations or Catalogs are not exactly unambiguous.

@jaymedina