Aggregation scopes

Scopes control which members contribute to an aggregation.

Aggregation functions need a scope to know how to aggregate measures. On the other hand, a scope cannot be passed when aggregating table columns or column operations: their aggregation will start from the facts.

CumulativeScope	Scope performing a cumulative aggregation.
OriginScope	Scope performing an aggregation at the given origin.
SiblingsScope	Scope performing a "siblings" aggregation.