Renaming particle matter interactions models and types
Following discussion from !466 (merged), the types of Particle Matter Interactions should be the name of the physics models. Then, CollimatorPhysics will be renamed as ScatteringPhysics.
I also propose to rename BeamStripping element as Vacuum in order to use more intuitive element names
This issue involve updating some regression-tests (see regression-tests#101 (closed))
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