Migrate to SQLAlchemy

[DavidStirling]

Moving away from sqlite/mysqlclient would help to unify a lot of the functionality and could also open up a pathway to supporting other database types. More importantly, pandas to_sql can handle any sqlalchemy connection whereas it can't work with mysqlclient. That function alone makes reading and writing tables much easier and would automatically resolve many of the errors people currently encounter with queries failing due to excessive length.