Add units support via the pint package

processing issue https://git.scc.kit.edu/jk7683/vre-language/-/issues/5

Several things will be left open for future changes/extensions:

1. the [] around the units
2. the implementation in pandas and numpy
3. units checks for function parameters and return values, see https://pint.readthedocs.io/en/stable/wrapping.html