Fixes #39.

I have refactored the data-classes a bit. I did not make as many changes as anticipated - I have mostly just moved work out of init and consolidated the get_from_table methods into a single method. It's more testable now.

I have also added docstrings in the numpydoc format.

Review Checklist for Source Code Changes

• Does pip install still work?
• Have you written a unit test for any new functions?
• Do all the units tests run successfully?
• Does adler run successfully on a test set of input files/databases?
• Have you used black on the files you have updated to confirm python programming style guide enforcement?