Linting, docstring, and implementation cleanups

[rkingsbury]

In this PR, I'm trying to expand the documentation of key classes, bring the linting and code quality standards into line with the latest pymatgen, and address a few small bugs and issues I've encountered. Comments welcome!

Changes:

• Add more documentation to concentration_matcher, Aqueous, and PackmolWrapper
• Fix a potential bug in concentration_matcher in which solvents and solv_ratio are not the same length
• Enable PackmolWrapper to handle paths with spaces in them (actually does not work fully)
• Add option to use vdW radii from pymatgen.Element, which cover nearly the whole periodic table
• Resolve all mypy linting errors triggered by the type hinting I added to selected docstrings. These are usually indicators of potential weaknesses in code, and fixing them tends to make the code a lot more robust

[rkingsbury]

@htz1992213 I know linting with black generates a huge number of changes that make it hard to review, so if you'd prefer I can put that into a separate PR and then build the documentation changes on top of it. Just let me know!

[rkingsbury]

@htz1992213 I know linting with black generates a huge number of changes that make it hard to review, so if you'd prefer I can put that into a separate PR and then build the documentation changes on top of it. Just let me know!

I answered my own question and went ahead and put the major linting changes in a dedicated PR #4

[rkingsbury]

Thanks for the comments @htz1992213. See what you think of my changes.