ST6RI-793 Transition source cannot be a feature chain

[seidewitz]

As reported in OMG issue SYSML2_-222, the derivations of the TransitionUsage::source and target properties given in the specification currently do not support the use of feature chains as the source or target of a transition. However, the derivation for TransitionUsage::target as implemented in the Pilot Implementation does already allow the use of feature chains as transition targets. Unfortunately, the implemented derivation of TransitionUsage::source did not allow feature chains to be used as transition sources, which was inconsistent.

This PR:

• Updates the implementated derivation of TransitionUsage::source so that it also consistently allows feature chains, presupposing the eventual resolution of SYSML2_-222.
• Updates the selection of the member action usage of the Transition from which the source is derived, to skip possible owned members that are features (like metadata usages) but are not action usages.