SMM12 — Move constraint on the “product” accumulator in CollectiveMeasure to CollectiveMeasurement.

When accumulator is "product", there must be one-and-only-one base measurement per measure, unless the units of base and collective are dimensionless. This is a constraint on CollectiveMeasurement, not on CollectiveMeasure.

Move constraint.

Strictly spoken, the constraint is too strict. Therefore, we move it indeed from Subclause 11.2 to Sub-clause 14.2, but not as constraint, but, in softer form, as a part of Semantics, meant as a possible idea or suggestion to implementers, but not strictly enforced by the spec.