Reorganize to library

[philzook58]

I've been finding PyRes quite intriguing and inspirational. Thanks for writing it! I think it is quite useful as a library if organized in that way. Subjectively, I think it is easier to see what are the main entry point files in this form also. This pull request has a suggestion for a minimal change to make installable via

python3 -m pip install -e .

or

python3 -m pip install git+https://github.com/eprover/PyRes.git

The package could be fairly easily submitted to PyPI once in this form too.

I have not fixed the Makefile to run the tests, They can be run via python3 -m PyRes.clauses for example, but I'm not sure hey can be called directly using the relative import form.

Another option would be to place the top level files into the PyRes directory so they are executable anywhere as python3 -m PyRes.pyres-simple

I wanted to see if this pull requests is of interest before I attempted anything further. It feels presumptuous to reorganize the files in someone else's project, so my apologies if unwelcome.