All Docstrings have been examined for typos and format, etc.
Small code change to set Selection Mode to "SEL" if operation is not
"C" or "P" to avoid possible errors when using other operations.
- Remove unused imports
- Add comments where appropriate
- Improve readability of variable names
- Re-implement a couple of functions using dictionaries instead of if-statements
- Results of Pylint & Black operations, plus some fixes to code.
