Fixed activeVolume for assemblies

[lobis]

• Fixes activeVolume definition not working properly with assemblies restG4#43
• Replaces Nevents by nEvents in RML files (Nevents still works) for consistency
• Replaces gdml_file by gdmlFile in RML files (gdml_file still works) for consistency
• Replaced "plate" by "circle" in generator, for consistency with Geant4 (also I don't think a plate is necessarily circular, its more like a "sheet")
• Other minor changes

[jgalan]

If we change the value of the parameter name, at least it should follow the standards. The corresponding metadata member is fGdmlFilename then the parameter should be gdmlFilename. I do not mind changing the metadata member name, but the naming should be consistent.

Then, if we include the line TRestMetadata::InitFromConfigFile() at the beginning of TRestGeant4Metadata::InitFromConfigFile, all the parameters matching their metadata member naming convention will be initialized with the RML values.

This could simplify the parameter retrieval in that method.

[jgalan]

I see now that gdmFile is actually the parameter, and gdmlFilename is formed internally. Still, if TRestMetadata::InitFromConfigFile is called, then we could initialize directly fGdmlFilename. Although the logic might need revision.

[jgalan]

We should probably throw gdml_file to adapt to the new naming conventions.

[lobis]

I see now that gdmFile is actually the parameter, and gdmlFilename is formed internally. Still, if TRestMetadata::InitFromConfigFile is called, then we could initialize directly fGdmlFilename. Although the logic might need revision.

I use explicit loading of the parameter to support both names for the parameter. I agree that the logic might need revision