Move unit system assignment for new well paths into RimWellPathCollection::assignUnitSystem, used by addWellPath and onChildAdded so Python-created well paths get the case unit system. Ignore unloaded and unknown-unit cases in RimProject::commonUnitSystemForAllCases.
Pasted and multiple-lateral well paths are copies that already carry unit specific settings, so skip unit assignment for them. Note: RimProject::commonUnitSystemForAllCases now ignores unloaded and unknown-unit cases and returns UNKNOWN only for mixed units, which affects its other callers.