Fixes recursion exception in davisb_putnamb_logemann_loveland

[spazm]

Describe your change:

• Fixes the recursion exception bug in dpll. Removes a case where recursion is called with unmodified symbols.
• Fixes recursion branch bug where both left and right actually evaluating right. Python arrays are copied by value, so both calls received the same modified model.
• Adds step comments
• renames vars to better semantic values
• pythonic clean-up around arrays.

• [ ] Add an algorithm?
• [X] Fix a bug or typo in an existing algorithm?
• [ ] Documentation change?

Checklist:

• [x] I have read CONTRIBUTING.md.
• [x] This pull request is all my own work -- I have not plagiarized.
• [x] I know that pull requests will not be merged if they fail the automated tests.
• [x] This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• [x] All new Python files are placed inside an existing directory.
• [x] All filenames are in all lowercase characters with no spaces or dashes.
• [x] All functions and variable names follow Python naming conventions.
• [x] All function parameters and return values are annotated with Python type hints.
• [x] All functions have doctests that pass the automated testing.
• [x] All new algorithms have a URL in its comments that points to Wikipedia or other similar explanation.
• [x] If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.

[poyea]

davisb_putnamb_logemann_loveland.py looks weird

[stale[bot]]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.