Update the XMI Loader implementation for the new structure of UML 2.5.1 #31
Description
As of UML 2.5, there are no longer separate infrastructure and superstructure metamodels, conformance levels or use of package merge in the UML abstract syntax. The XMI loading implementation should reflect this and be based on the latest UML version, 2.5.1, rather the old version 2.4.1. Note that, while UML 2.5 also introduced a new package structure, at the metaclass level, the fUML subset of the UML 2.5.1 metamodel is the same as the fUML subset of the UML 2.4.1 metamodel as fully merged at level L3, so XMI files from previous version of UML can still still be loaded.