Avoid unnecessary repetitions of import-statement for code lists

[peter-vermeulen]

Look for the yellow marked repetions in the XML schema text. (code 000029 is assembled list for codelistresponsibleagency. It looks therefore that for each usage of the codelist it is imported without taking notice of the fact that this codelist maybe has already been imported)
We see also repetitions in the generated generic code lists (generated package ”generic”). Here we would expect a long list with XML schema’s for each enumeration specified in the UMM model and used by inheritance by the enumerations stereotyped <> which also need to have their own XML schema. But no repetitions of one and the same code list.

[peter-vermeulen]

The file https://github.com/peter-vermeulen/ebix/blob/master/org.ebix.umm/src/org/ebix/umm/templates/xsd/BdtLibraryExtension.xtend contains the function allReferencedCodelists.
This function creates the list of codelist, and sorts them alphabatically, but doesn't avoid or remove duplicates.

Changing the ArrayList into a HashSet is probably enough.