ENH: Plug-in system for databases, possibly remove SQLAlchemy

[NickleDave]

plug-in system for databases

• we should provide some plug-in system that allows for using other databases
  • this will probably be an "extras". We should avoid entry points or other developer-focused things and have it be like a registry with decorators instead
  • a proof-of-concept would be using the plug-in system to import: SAP database, Koe database, Luscinia database

remove sqlalchemy?

• sqlalchemy is a heavyweight dependency I think. It seems like it's a big drag on the install
• we don't have a clear story/workflow for sharing datasets right now, this will depend in part on how we implement ENH: have functions accept vocalpy.Audio, return vocalpy.Spectrogram, vocalpy.LineSegment, etc. #48 ENH: Add LineSegment + Sequence + Box classes, to be returned by segment algorithms #49 ENH: Rename Unit to LineSegment (?) #50 ENH: Add Sequence and Boxes classes, to be returned by segment algorithms #51 (see https://github.com/vocalpy/vocalpy/tree/refactor-segmentation)
• if sqlalchemy really is a drag on the install, we should revise the workflow so that it's pure SQLite, and remove the dependency on SQLAlchemy