Make unit selection more fine-grained

[stfnp]

Currently the unit of a parameter in the bow model depends on the group/quantity that the parameter belongs to, e.g. "length", "force", "mass" or whatever. But sometimes it would be convenient if two parameters of the same group could have different units. For example, draw length in inches while all the measurements are still in mm. Or arrow weight in grains and the other masses still in grams.

I envision something where there are the unit groups, as it is now, but you can expand them to see and edit the individual parameters that belong to the group. Setting the unit of the group applies to all the parameters in the group, but you can also expand the group and set individual units, which then override the group setting. That way each unit is configurable individually but whole groups can also be changed quickly.