compton-convgen: Misc: Clean up. The commit brings no change to the
functionality of the script.
- Partially fix PEP 8 compliance:
- Place imports on separate lines.
- Replace leading tabs with 4 spaces.
- Add docstrings to classes and functions.
- Surround top-level function and class definitions with two blank
lines.
- Remove spaces around keyword arguments.
- Move all statements to separate lines.
- Break some long lines into several lines.
- Remove trailing semicolons after statements.
- CGError: Use functionality from the base class Exception to store the
description, instead of the custom logic.
- CGInternal: Remove, as it is unused.
- Hide the internal function gen_invalid() and args_readfactors() by
prefixing their names with an underscore.
- Move the module-level command-line handling code to two new
functions, _main() and _parse_args(), and only execute if running in
the main scope.
