Resolve "PluginElements: use unit meter internally to avoid conversions"

Closes #307 (closed) and #539 (closed)

This MR deals with the cyclotron PluginElements (CCollimator, Probe, Septum, Stripper) and BeamStripping.

• Convert unit dimensions of input directly from mm to m
• The bunch coordinates (bunch->R) in the plugin elements are changed from mm to m (closes #307 (closed))
• Septum uses internal LossDataSink instead of PartBunchBase one, making it obsolete.
• Cleanup PartBunchBase
• Add missing header to PluginElements