Make it easier to update Traveler

[AntonPetrov]

We need to develop an easier way to update Traveler in R2DT. Currently there is a set of test sequences and their corresponding diagrams. The tests only pass if all newly generated diagrams are identical to the examples, which gives confidence that no accidental changes affecting the layouts were introduced. However, any upstream changes in Traveler require updating all or most example diagrams which is a manual process.

I suggest to implement:

• a new r2dt.py command to compute new diagrams for the example sequences
• organise the new and old diagrams in a way that makes it easy to compare them visually (in a folder with consistent names or as an HTML report)
• once the layouts are verified, another new command should replace the example diagrams the new ones.