Python Packaging

[jwaldrop107]

Is this pull request associated with an issue(s)?
No.

Description
The PR makes changes to move towards packaging FriendZone as an explicit python library by adding the pyproject.toml and related updates to .gitignore.

Changes are also made to the CMake build and the friends.py module to check for Python dependencies in a more Pythonic fashion.

[jwaldrop107]

@ryanmrichard and @zachcran, this has morphed into a larger refactor than I originally intended. I've tried to update the code to differentiate between the interfaces (ASE and MolSSI) and backends (just NWChem for now), and to behave according to which combinations are available.

[zachcran]

Overall, looks good. The most important suggestions are for the testing strategy, but I also came across a few nits to pick at in the other files. Feel free to ignore some of these suggestions, but please reply saying so and don't resolve the thread. Otherwise it can be really hard to find them again sometimes or it might seem like you missed it!

For a few of the comments, I may not have pointed every instance of the issue, so make sure to do a once-over checking for it elsewhere if it seems like something that might pop up in other places.

[zachcran]

LGTM