Requirement for experimentalContextQualifier in ExpVarFIPropostion

[mbrush]

Given that Propositions are not defined to be study-specific, I would assume that we do not want to require the `experimentalContextQualifier ' attribute in the ExpVarFIPropostion profile, as is currently the case (see here). I suspect this may be unintended result of deriving the Proposition definition from the former StudyStatement definition, where it made sense to require this attribute.

This qualifier can be used in Statements that are describing study-contextualized functional Impact assertions, and required in this context. But IMO should not be required at the Proposition level.