Click for kicks

[JedMeister]

This was initially and primarily a practical experiment with click - which is an alternative cli argument parsing library. In the case of Pool, I'm replacing argparse (which in previously turn replaced optparse).

But there were still a few typing issues, so I fixed them first. Then after implementing the click stuff, in the interests of organization and readability, it seemed splitting out the non-core cli functions into separate files (in pool_lib) was a good idea.

Overview of massive refactoring:

• finish/fix typing
• implementation of click to manage cli options (replaces argparse)
• refactor/reorganize code - new files/paths:
  • rename pool_bin -> pool
  • move pool info functions to pool_info.py
  • move general functions to utils.py
  • move remaining non cli arg related func(s) to extras.py (not sure what these are for, perhaps build-info?)

Whilst this code looks ok and passes typing checks (pyright) and style linting (pycodestyle), at this stage it is completely untested...