Complete generalization of HVAC topology tracing

Partially reverse implemenation of Ford–Fulkerson algorithm in favor of  a more
simple depth first search. The old strategy contained notion of flow
capacity which is not required here.

This approach is only used by Loop::demandComponents but could be
applied to all HVAC system tracing. Now Loop::demandComponents is
sophisticated enough to trace systems that contain plenums (splitters on
splitters) and plenum related unit tests now pass.