Fix the way FluxCat is storing its catalogues.

[ljgray]

Thinking about HFBcat, if we want to make this its own package, we really need to fix the way FluxCat is storing its catalogues (see the problem I addressed here for HFBCat: chime-experiment/ch_util#86 ). I think the easy solution is to move these class variables its using to store the catalogues it loads at import time out of the class and turn them into module-level attributes.

Originally posted by @ketiltrout in #1 (comment)