Moved .download functions to .db

[MJWeberg]

Phase 2 of search and download refactor. Mostly internal changes that should be invisible to general users.

Summary of changes

• Moved download_db() and download_hd5f_data() from eispac.download to eispac.db
• Moved low-level time conversion functions (tai2utc and utc2tai) from eispac.download.convert to eispac.util.convert
• Replaced calls to the old EIS_DB class with EISAsRun in the eis_catalog GUI
• Added pass-through functions to eispac.download for the moved functions and added deprecation warnings. Very few (if any) users will see these warnings since no one was likely calling these functions directly.
• Small update to the API documentation (may take a while before ReadTheDocs updates)
• Assorted very minor bug-fixes