Pylint fix

[chiefeu]

1. initially used 'black' (pip install black) for basic fix.

2. then manually modified each python file based on the pylint output.

   • added module, class, function docstrings
   • removed/commented out unused variables (in for loops, we rename the variable to _ )
   • removed/commented out unused imports
   • reorganized imports
   • fixed line-too-long warnings
   • fixed naming conventions suggested by pylint
   • fixed missing explicit default parameters
   • etc.

3. other pylint warnings (too many branch, local variables, statements, etc.) needs to refactor the whole code to be fixed

   • src python files for further refactoring.