Usability changes and fixes.
- "Duplicating" the selection into a cell now adds the cell to a model.
- Make NormalFeature modify the selection instead of creating a new cell. This way it is easy to try new values. Also, fix point/cell-normal arrays in a way that lets Duplicate work.
- Implement a proximity-based selection operation.
- Generalize pqNormalFeatureReaction into pqFeatureReaction and use it for the proximity selection operation as well.
- Documentation for selection operations.