Bugfix for BlockAssembly rich comparison methods

A bug in BlockAssembly.vertexAttributesEquivalent(self, other) made comparison methods return false negatives. This came from incorrect registration between assemblies with different numbers of blocks. Specifically, if self < other then the centers of the two assemblies will be different (in general). I fixed this by centering other on the subset of blocks in other that are also in self (this is guaranteed to include all the blocks in self if the self < other).

I also fixed a bug in compute_stats.constructGoalState() that was causing it to return the mirror-image of the actual goal state.

Finally, I modified BlockAssembly.draw(self, file_path, state_index) to render a typical pyplot figure instead of writing to file if file_path == None.