Generalize (partially) HVAC topology tracing

This work treats HVAC topology as a graph and uses a generic recursive algorithm
inspired by the Ford–Fulkerson algorithm to trace HVAC all HVAC
components between source and sink.  This algorithm is still being
developed and implemented behind the existing API like
Loop::demandComponents() but should make implementation of this code
much cleaner.

http://en.wikipedia.org/wiki/Ford–Fulkerson_algorithm