refactor database usage

[Godley]

I'd like to abstract away from using sql queries, probably using SQLAlchemy. Reasons:

• Some people don't like SQL or know it (Idk how)
• Some of the statements are pretty huge, particularly the join ones
• They make it hard to fit the PEP8 regulations on line length and making it fit the line length means having statements span about 8 lines
• It's not very pythonic and it's a bit hacky just having them thrown in
• I've made dictionaries of sql queries to get around code duplication which is really just not nice

[Godley]

Closed with last PR, moved to using SQLAlchemy's query objects. Not using the ORM because it can't handle 3 way joins (such as instruments-clefs-pieces). I've also made it raise an exception if each piece doesn't have a clef or key, which may or may not be a good thing, but just simplifies the process of adding instruments and clefs/keys.