ST6RI-866 Substates inherit two "this" features

[seidewitz]

This PR proactively implements a resolution to the following issue, even though it has not yet been voting on by the SysML 2.1 RTF.

• SYSML21-311 Substates may potentially inherit "this" twice

This problem actually manifested itself in the Pilot Implementation such that substates always inherited to this features. The problem was resolved by updating the declaration of the redefinition of this in Actions::Action::subactions to explicitly redefine the inheritable feature from each of the types specialized by subactions:

action subactions: Action[0..*] :> actions, subperformances {
    ref occurrence :>> Action::this, actions::this, subperformances::this = (that as Action).this {
    ...
}