Unify CLI to either argparse or click

[NichtJens]

• The slight variations between the two help outputs are annoying.
• Also, get rid of the hand-made subcommand logic in bs.py and use a built-in one.

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Docs for subcommands:

• argparse

  • https://docs.python.org/3/library/argparse.html#argparse.ArgumentParser.add_subparsers

• click

  • https://click.palletsprojects.com/en/stable/quickstart/#nesting-commands
  • https://stackoverflow.com/questions/34643620/how-can-i-split-my-click-commands-each-with-a-set-of-sub-commands-into-multipl

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What uses what:

• click

  • avail
  • receive
  • stats

• argparse

  • config
  • create
  • h5
  • simulate
  • (utils/create_test_db)
  • (unused/bsavail)
  • (tests/perf_dispatching_layer)