Fix parameter units being silently changed to SI units

[henrikjacobsenfys]

This was written partly by Claude under my guidance.

Handles the issue mentioned here: #300

The solution is twofold:

1. store the string users give when supplying a unit, and use that string rather than the scipp-generated one. This means users get the unit they asked for, not whichever unit combination the underlying library thinks it should be
2. strip numbers only when the unit is purely a number, e.g. in the m/mm example.

For 1, it also handles e.g. when parameters are added or subtracted from other parameters.