ENH: move reusable methods from cosmic_gps to methods

[jklenzing]

A lot of the methods are reusable for other potential CDAAC instruments. We could move these from the cosmic_gps instrument into a methods file that all can pull from.

• download - ENH: generalize download methods #30 - test with cosmic2 ivm
• list_files - ?? - test with cosmic2 gps
• load - ?? - test with cosmic2 gps

[rstoneback]

Some supporting work has been done over in pulls #17 and #18 to make it easier to move some methods to a different file. I suspect this issue will get addressed, in part, as part of adding cosmic-2 in #4. The additional instrument will make clearer the areas that are common within CDAAC, and not limited to a particular satellite constellation.