Fixed bug in pairs generation in graph matching algorithm.
Fixed bug in pairs generation in graph matching algorithm.

If you provide an initial mapping to the VF2 algorithm, it must make sure that
these are not included in the proposed pairs. Once in a while the previous
code would let one slip through. The fix eliminates this and therefor makes
the algorithm more robust.

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