Refactor code to meet pep8 style guides and to streamline reading

[robwalton]

Hi Nikola,

Thanks for sharing this. I've looked around for a well tested and commented prolog implementation in Python and landed with yours. I will be calling it programatically and so dived in a little to check out the source.

I've made what look like a lot of changes to make the code meed style guidelines and to streamline the way it reads a little. Basically this pull request:

• converts comments to doc strings
• reformat to meet pep8 (using black)
• fix linter warnings
• move to pytest unit testing
• Streamline Parser iteration and remove duplicate code
• change package names to meet pep8
• make some things private or static to improve readability
• add some usage instructions to the README
• moved editor.py up to top level so it can be run with python from the root directory

These changes do not change the logic in any way! I hope you think they are mostly improvements.

[stefan-c-kremer]

This is an impressive contribution. Thanks, @robwalton !

[photonlines]

Awesome - thank you for the contribution Rob. Amazing work! (Also, apologies for the delayed response - I didn't realize you made a pull request until today!)

[robwalton]

No worries. The unit tests are not quite right in the pull request and are too loose. I think the original may her been a bit loose too. Have slightly moved on from this, but will see if I can find the branch where I fixed them!