Do more type checking

[jkomoros]

Especially with things like #100 it gets more and more likely we break something when refactoring.

Ideally we'd get to a point where every method had type hints and we flagged errors as we were working on it (e.g. in VSCode: https://www.emmanuelgautier.com/blog/enable-vscode-python-type-checking)

• [ ] Turn on basic error checking
• [ ] Turn on 'strict' error checking
• [ ] Use @datastore
• [x] Make importers all descend from a base importer abstract class
• [x] Convert all Importers to be descendants of BaseIMporter
• [x] Switch the convert.main to have a map of BaseImporter
• [x] Have Exporters inherit from NullExporter