Support wrapping `ValuesBounded`

Description

Supersedes !69 (closed)

• Decision is made about whether we need to support wrapping this class at all. I think the answer is that we do, but maybe there is a way we can avoid this by doing something else.
• If we decide that we need to wrap this class, we should be able to do the following:
  • create a ValuesBounded class in Python using Pint quantities only
  • have the units and magnitudes appear correctly on the Fortran side
  • error is raised if ValuesBounded.values and ValuesBounded.value_last_bound can't be converted to the same unit
  • the values and value_last_bound attributes are exposed in Python as Pint quantities
  • the units attribute setter is not exposed in Python (doesn't make sense on the Python side)

Checklist

Please confirm that this pull request has done the following:

• Tests added
• Documentation added (where applicable)
• Changelog item added to changelog/

Closes #41 (closed)