Optimize coordinate conversions of the Eigen stepper

...in which I attempt to resume integration of Nicolas' internship work into ACTS.

Now that Andreas has reworked the Propagator so that it does not run a coordinate conversion on every stepper iteration, these optimizations are much less useful than they used to be. But we may need coordinate conversions to be fast again in the future (e.g. if someone introduces a very useful observer or abort condition which absolutely needs to have the track parameters in a certain format), so I think this might still be worth merging.

Confused reviewers are advised to explore this MR commit-by-commit, as the changes have been neatly separated into categories that make sense in the commit history. Some commit messages will also get a bit more into the details of why some changes are valid (e.g. removal of norm_vec in global-to-curvilinear coordinate conversion).

Integrating this will close ACTS-340.