Allow dimension configuration for Event Nexus Reader

[OwenArnold]

Dimension properties are currently hard-coded for this reader. We already have widgets that can do this sort of configuration, but in order to use them geometry xml must be obtained from imddimension types. Currently generation of the xml is the responsibility of the MDGeometry type, which is not used for event-based visualisation.

1. Extract the geometry xml logic from MDGeometry
2. Enable dimensional configuration for vtkEventNexusReader

[OwenArnold]

This issue was originally trac ticket 2995