Improved as-run db management

[MJWeberg]

Improved tools for locating, downloading, and searching the EIS as-run catalog

Summary of changes

• Created new db submodule
• Added eispac.db.find_eis_cat() function for automatically locating the user's local copy of the as-run catalog
• Added eispac.db.get_remote_db_modtime() function for checking the modification date of the catalog on a remote server
• Added eispac.db.EISAsRun() class. Updated version of the eispac.download.EIS_DB() class with cleaner search results (now returns an Astropy Table!) and methods for locating and updating the catalog itself.
• Small changes to the doc strings in the eispac.download module.

This is phase 1 of a larger project to refactor the search and download functions in EISPAC. Future changes include:

• Moving all functions in eispac.download to the db and net submodules and adding depreciation warnings and passthrough functions for anyone using the old download submodule (these functions are mostly used internally by the eis_catalog GUI, but it is possible some users have been calling them directly).
• Replacing EIS_DB with EISAsRun in the eis_catalog GUI
• Expanding the sunpy Fido client to use the as-run database (if available). This will allow users to search by any of the metadata in the catalog (e.g. study ids, raster xcen/ycen, wavelengths, etc...) rather than just date_obs.