Create new PCC operation that allows application-maintained subset

[arthur00]

Encode a Projection+Subset, except not using a query/predicate, so that all objects of that type does not need to be checked. A common use for this is Node/BusinessNode/SimulationNode on the nodesim application. Ideally, get(BusinessNode) should return only BusinessNode, and not Node. BusinessNodes are created, modified, and deleted by applications, and never become anything but BusinessNodes (i.e. their implicit predicate of IsBusinessNode is ALWAYS true once set).

[arthur00]

Partially addressed in commit 42c5d5f. Closing for now, until a necessity arrives.