Reconcile graphs before testing (to deal with random UUID)

In NIDASH call on November 3rd ([minutes](https://docs.google.com/document/d/175OESAn7jsupAUT_GRq5JwgMmu0YLQPQWrRKfOrCPRY/edit?usp=sharing)), we discussed the possibility to use random UUID as identifiers (cf. also #109 and #211).

This pull request updates the testing procedure to reconcile graphs before comparison. This would allow to determine that two graphs are identical even if they do not use the exact same identifiers.

This code is used at incf-nidash/nidm-results_fsl#16 for testing.