Feature request: option to disable units canceling?

[emlys]

Hello,
In some situations, units should not cancel out in division. For example, soil erodibility is measured in t * ha * hr / (ha * MJ * mm). By convention the hectares are left on the top and bottom, to emphasize that it measures soil loss (in t) per unit area (in ha) per unit of storm erosivity (in MJ * mm / (ha * hr)).

The option to instantiate a pint.Unit that evaluates without canceling would be useful in this situation. Something like:

>>> non_canceling_unit = pint.Unit('t * ha * hr / (ha * MJ * mm)', cancel=False)
>>> print(non_canceling_unit)
hectare * hour * metric_ton / hectare / megajoule / millimeter

[MichaelTiemannOSC]

I would love to make tons of things non-cancellable. When computing emissions intensity, it would be great if t CO2 / t Steel did not cancel to CO2 / Steel. PLEASE!

I'm more than OK when t CO2 / t CO2 becomes dimensionless. It's just when substances are different I don't want the mass unit to disappear.

[hgrecco]

Please take a look at #551 I would love to merge something in this direction, if someone wants to generate a PR