Use pint for unit conversion

Unit conversion should probably be handled with [pint](https://pint.readthedocs.io/en/stable/).

It's built into both [pint-xarray](https://pint-xarray.readthedocs.io/en/latest/index.html), [xclim](https://xclim.readthedocs.io/en/stable/notebooks/units.html#Unit-handling) and [MetPy](https://unidata.github.io/MetPy/latest/tutorials/unit_tutorial.html) - given that pint-xarray in still in the early stages of development I think xclim is the easiest to use right now (see https://github.com/AusClimateService/unseen/commit/512ab14f96660cac229f83d3960b344bd6cdcdd9).

No file or test is named. Start by reading the linked pint, pint-xarray, xclim, and MetPy material plus commit 512ab14f96660cac229f83d3960b344bd6cdcdd9; clarify which unit-conversion paths are in scope before defining completion.

Written by the indexing model from the issue text.