Add `type` column to mesh warper CSV import/export

[adamkewley]

Where type is one of: Landmark, Semilandmark, NonLandmarkPoint (non-participating)

This is so that the higher-level warping workflow (#802) is able to read a single CSV per mesh (e.g. from Landmarks/MeshName.csv) and have all the information it needs to start warping other stuff in the model.

The file must also be importable into the mesh importer (#803), because users are going to also use the landmarks as part of defining frames - both in the importer (#802) and in existing opensim models (issue not made yet).

And, ideally, this would be done in a backward-compatible way (i.e. the CSV reader should work with older CSVs that have no type column).