Updated use of callDepth in graph isomorphism.
Updated use of callDepth in graph isomorphism.

The callDepth variable is now initialized with the number of vertices in the
smaller graph *minus those already matched in initialMap*. When callDepth is
zero, then we should have a complete isomorphism or subgraph isomorphism
match. The code has been refactored accordingly.

This lets us use the rapid test callDepth == 0 to check if we are at the
end of an isomorphism search, which provides a small speed boost.

enochd/RMG-Py,nickvandewiele/RMG-Py,comocheng/RMG-Py,faribas/RMG-Py,pierrelb/RMG-Py,pierrelb/RMG-Py,KEHANG/RMG-Py,nyee/RMG-Py,nickvandewiele/RMG-Py,faribas/RMG-Py,KEHANG/RMG-Py,chatelak/RMG-Py,jwallen/ChemPy,enochd/RMG-Py,comocheng/RMG-Py,nyee/RMG-Py,chatelak/RMG-Py