Docstrings cleanup

[mattkwiecien]

We don't have a consistent style for our docstrings. This was previously causing warnings in the sphinx auto doc and may be the underlying cause of #384.

Acceptance Criteria

• Decide on either Google or NumPy docstring convention
• Add a linter that we can include in our pre-commit checks (flake8-docstrings seems like it may do the trick)
• Update docstrings to be compliant with whichever convention we choose.

[marcpaterno]

We have decided to use Sphinx-default format for docstrings. The documentation is available at https://www.sphinx-doc.org/en/master/tutorial/automatic-doc-generation.html.

Note that because we are using type annotations in the code, we do not specify types for arguments or for return types for functions or methods; these are deduced from the type annotations.

We are using flake8-docstrings.

We have not yet updated all documentation to follow these standards. We are closing this issue to record the decisions, and will create another issue to cover updating existing code to these conventions.