Create NXprocess_mpes?

[lukaspie]

• Should there be a named calibration for each named axis in NXdata? We could store all named processes in NXprocess_mpes (with bse class inheritance) and then align with NXdata_mpes
• Other options: add enumeration to NXcalibration to make all foreseenable observable be selectable (maybe via enumeration)
• In NXprocess: store bin size for rebinning process (effective acceptance of a bin)