Review and make use of dot validator in DotDiffEngine

[alfonsodelavega]

There is a dot validator available for the dot diff engine that is not currently being used. It comes from the original dot diff prototype.

As I am periodically finding (and then forgetting) the same "bugs" or missing information in the m2t transformations (e.g. label names), it seems that checking and including this validator in the execution pipeline makes a lot of sense.