Bugfix 2675 move to pyproject.toml

[John-Sharples]

• Move to pyproject.toml for build and install
• removes setup.py and references to it in Dockerfile
• adds run_metplus as CLI script
• Note this introduces a duplication of the version number, once in pyproject.toml and again in VERSION. Perhaps not ideal, but I couldn't make setuptools fetch the version from VERSION.

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