ST6RI-585 Message ends are parsed incorrectly

[seidewitz]

This pull request corrects bugs related to the parsing of message declarations.

1. A message declaration is parsed as a FlowConnectionUsage, which, as currently implemented, must have end features that are ItemFlowEnds. However, message ends where previously being parsed as regular Features. This is now corrected so they are parsed as ItemFlowEnds.
2. Message ends do not have itemFeatures (i.e., target output and source input features). However, the derivation computation for ItemFlow::getItemFeature was throwing an index-out-of-bounds exception for a FlowConnectionUsage with ItemFlowEnds without itemFeatures. This is now corrected.