Bugfix: isomorphism check was not strict enough

Some non-isomorphic graphs were erroneously considered to be isomorphic. It could
happen that two different nodes were not compared as they've already been marked as
visited by some other comparisons. With this commit, a node comparison will be
skipped only if exactly the same nodes were already compared before.
Merge pull request #212 from Stoeoef/bugfix-check-nodes-twice

Bugfix: isomorphism check was not strict enough