Implementation of relative_time_constant

For compatibility with the relatively exchangeable (in certain cases) integer_constant, real_constant, relative_time_constant (see librecube/prototypes/python-pluto-parser#34 (closed)), it would be convenient to have a relative_time_constant transform to a ureg/pint quantity. A full implementation of the transformer method for relative_time_constant will be tricky because of the rather large variety of different combinations for creating the time interval.

This is complicated by the fact that currently, Pint interprets 1h as 1 Planck constant (see https://github.com/hgrecco/pint/issues/82 and the newer https://github.com/hgrecco/pint/issues/719).

One option would be to detour around python's datetime for parsing the incoming string correctly (if that is possible in a flexible way) and feed this into ureg() with a known unit (e.g. s).