Start to introduce zstart/zend and change loops (updated)

[ZedThree]

#1448 + next

The merge commit had a lot of conflicts, hence new PR.

I think maybe we should undo the changes to ShiftedMetric -- at least for now, we probably want to transform all of Z.

[dschwoerer]

What is the best way to move this forward? Is it worth trying to resolve the conflicts, or is it easier to start again?

Also, should this be split in several PRs, to make it easier (and thus faster) to review?

[ZedThree]

We have a new RSE starting next week, this will be on their to-do list, but it might be awhile before they actually get to it.

I haven't looked at how bad the conflicts are, but I suspect that will determine how we actually proceed.

Multiple PRs might be better, but the hope with the new RSE is that we'll have a bit more bandwidth to review PRs and get them merged faster.