PluginElements: use unit meter internally to avoid conversions
For consistency and #45 the plugin elements (septum, ccollimator, probe and stripper) should use unit m instead of mm. This will remove conversions and is much less error-prone.
For consistency and #45 the plugin elements (septum, ccollimator, probe and stripper) should use unit m instead of mm. This will remove conversions and is much less error-prone.