Check that properties exist-- leftOuterJoin tested and working by itself.
Made partial_leftOuterJoin so that the relation mapper may be called on a per-instance basis and may be operated by a stream.  The stream operator can exist separate from any one in-memory relational operation based on declarative dependencies of our operation= that it is O(L*R) and each L is dependent on all the 'right' (aka child, or R) joins.
Fixed tests and algorithm to work as an outer, rather than an inner, join.