Junction: Now handles its virtual pin states via the Stateful interface
SourceFixed: No longer implements stateful.  (Doesn't make sense)
Stateful: Added orState()
Component: toString() now overriden to call getName()
ComponentPassive: Added default orState() implementation
Trace: Pin references are now private
SpatialGrid: Moved createIdentityHashSet() into Util, needed elsewhere
AffectedPathSet and associated functionality moved into Net class
Minor changes to Trace and Junction
Component: Made simStep public
Net: Now populates writeables list correctly
Simulation: Redesigne; implemented levelisation-based algorithm.
Should now be able to handle any combinational circuit.
TestHalfAdder: Updated to match new sim structure
Changed sources so reset() has no effect on their state