model: Operation should be a Namespace with three NameSpaceSets for its OperationVariables

This implementation has been reverted in 0fee9b12. We are currently awaiting feedback on this.

If this class stays in the metamodel, we could remove OperationVariable in our implementation and attach SubmodelElements to the Operation directly, so we can make Operation a Namespace.
Adapters would then just "emulate" OperationVariables.