Harmonize CLI with argparse

[woutdenolf]

• Modernize the CLI by using argparse instead of getopt.
• Implement common behavior only once (e.g. "--help" and multiprocessing.freeze_support): Edit: not possible (see Refactor global state on import #1197)
• Add unit tests

[woutdenolf]

Looking at the current multiprocessing.freeze_support() calls it needs to be done before importing anything instead at the CLI entry point. This can only mean that importing in PyMca creates global state. Creating global state on import never a good idea, it should be done lazily.