alternatives, criteria, categories: now maxOccurs=1. Same for xxxSets, and they must have an ID

All these six elements may now appear only once in a XMCDA file.  They
hold the so-called «referenceable objects»: objects that may be
referenced by their id within the same XMCDA project.
methodMessages renamed to methodExecutionResult. It now has a status indicating the status of the execution of an method/algorithm