163 database tweaks

[astronomerritt]

Fixes #163.

• AdlerData now correctly overwrites database values.
• The database/table is set up so that there will only ever be one entry for each ssObjectID: i.e. ssObjectID is the primary key and will be unique.
• On writeout, AdlerData tries to do an INSERT, and if that fails (because the ssObjectID exists), it will instead UPDATE the values in the specified columns.
• Wrote some unit tests to make sure this works as expected.

I also changed the timestamp column to an MJD and cleaned up the comments in the unit test (sorry!) because I couldn't see what I was doing 😅

At some point I may shift the database code out into its own class, especially when it comes to writing to the MariaDB database on the cloud, but that can happen later.

Review Checklist for Source Code Changes

• Does pip install still work?
• Have you written a unit test for any new functions?
• Do all the units tests run successfully?
• Does adler run successfully on a test set of input files/databases?
• Have you used black on the files you have updated to confirm python programming style guide enforcement?

[jrob93]

Looks good! Apologies for the weird _fixed vs fixed spby error. It was literally just something that worked up until sbpy==0.5.0. I now know _single_leading_underscore: weak “internal use” indicator!