Draft: #378 CIF data-based synthesis variable ordering and reordering in one go.

• The main idea is that the initial variable order and subsequent algorithms applied to it, are now one configuration, and they are applied in one go. This is a step in #378 (closed), towards also adding a new option that can create more complex such configurations, but that can be applied in the same way.
• Probably easiest to review per commit, but there is some shuffling involved that in hindsight I could have maybe done in more separate commits.
• The variable ordering helper is now always created from the model order. This may change the variable order. For the test models, I checked each change manually, and found it to be different but equivalent. I'm running the benchmark models now, to see if/how it affects them. I'll post a separate comment about it. But the merge request can be reviewed already.
• Some other smaller changes, see commit messages of separate commits.

Addresses #378 (closed)