Add `Attach to`/`Expressed In` advanced option(s) to CSV point importer

[adamkewley]

This issue was broken out from #149 because it's non-essential for shipping a minimal version of #149, but it is a useful addition that should be added when time allows.

This was brought up in discussions with @emmccain-uva, and the feature that was suggested is:

Should there be a feature in the mesh importer's Import Stations from CSV (#149) that enables importing data that is expressed w.r.t. some other frame in the mesh importer scene (additionally, an option to attach to a scene element, but not express it w.r.t. it?)

The general feeling was that it would be a very useful feature for the future but the version of #149 that we were discussing had enough features to move forward and that this addition shouldn't necessarily block that one from shipping.