xarray units check: check unit compatibility instead of units itself

When pushing data to outputs, the associated units need to match the output specifications at the moment.

In my opinion it would be better to add options convert_units and only_check_units_compatibility (both false by default) to data.check to convert compatible units in-place and to call this routine with check(...,only_check_units_compatibility=True, convert_units=True) in data.to_xarray in order to get the desired units. This would then also solve the problem in Output.push_data

pint provides a routine Unit.is_compatible_with to check if two units are compatible (link).

Maybe we could also only check compatibility (i.e. same dimensionality) in data.check by default, so we don't compare the units directly.