Use `setuptools_scm` to set version

[ZedThree]

I'd really really like to completely remove all the hardcoded version numbers throughout the project. I discovered recently that you can run setuptools_scm from the command line, so we could run that in CMake (and chop of the bits CMake doesn't like).

We'd want a fallback if setuptools_scm isn't present, but that could be from a file that setuptools_scm generates -- we could do that in CI.

[dschwoerer]

The issue with setuptools_scm is, it does not work on the master branch. It always bumps the patch level, and there is now way around that. That is the reason we did not adopt it so far.

While we are at it, it would be nice to print the full version when running BOUT++.