trackframe: Document or delete coredata

[katrinabrock]

https://github.com/livingingroups/travelpaths-devel/blob/336dc1f264a9f41aea19a86b08aa8c2461a834aa/pkgs/trackframe/R/backtransform.R#L88

I'm not sure exactly the purpose of this. I see that it's implementing this method: https://www.rdocumentation.org/packages/zoo/versions/1.8-14/topics/coredata but unsure how it would be used. I think we should either properly document it or remove it from the package.

[katrinabrock]

@FlorianSchwendinger can you prioritize this one? I think it's now causing check() to fail:

https://github.com/livingingroups/travelpaths-devel/actions/runs/20137993644/job/57796746363

[katrinabrock]

I just removed it in my last PR. Seems like a good idea to put it back in, but not so urgent.

[FlorianSchwendinger]

To the coredata it is not that important, I was just trying to have consistant behavior to ts / xts. Added a new proposal with docs to
https://github.com/livingingroups/travelpaths-devel/blob/feat-coredata/pkgs/trackframe/R/backtransform.R .