DOCS: Check spelling in docstrings

[asoplata]

Problem: Currently, our automated spell-checking tests ( https://github.com/codespell-project/codespell ) only check for common misspellings. This means that occasionally, our code and documentation can contain uncommon spelling errors (example: https://github.com/jonescompneurolab/hnn-core/blob/master/hnn_core/parallel_backends.py#L597 ).

Solution: We need someone to help with checking our current docstrings etc. for typos, misspellings, and grammar errors that are very confusing. This doesn't have to done for every docstring every where, but any amount of help is useful.