Make HISql object to write data to SQL database about housing-insights

[salomoneb]

Per discussion with Neal, assigning this to myself.

[salomoneb]

@prisalex and I worked on this today. We sketched out the blueprint for the full HISql object and wrote some preliminary code for the DataSql subclass. I committed our work to my forked repo in an add-datawriter branch. We had a few questions about how we'll be reading from and updating information in the database, but I'll put those through when they're more organized.

I'm going to try to work on the DataSql object some more tomorrow, but may not be able to pick it up again until Saturday.

[NealHumphrey]

@salomoneb and @prisalex - great! Reminder, I'm out of town for the week starting this afternoon (ski trip with my dad). I'll have my computer and will try to stay on top of Git and Slack messages, but will be slower to respond.

On reading and updating, not sure what your questions were, but here's what I had in mind:

• python/housinginsights/tools/dbtools.py has functions to get SQLAlchemy engine and connections. Feel free to move these into object oriented structure, though might be more flexible to leave them where they are and have the HISql object call them.
• Make sure to include a 'db_choice' parameter for HISql with default of 'housinginsights_local' or somesuch; the dbtools.py has ways to choose between local and remote.
• Obviously, we want to use meta.json for the appropriate field names and variable types to create the table if it doesn't exist yet.
• ManifestSql writes one row at a time. Create the table if it doesn't exist yet.
• DataSql uses COPY FROM to append the whole file to the bottom of the table.

As always, adapt as needed.