ITS: template Traits and TimeFrame

[f3sch]

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[davidrohr]

Looks OK from my side, but I would ask you to check that is has no performance / stability regression when running on the EPN GPUs.

[mconcas]

Hello @f3sch,

The TimeFrame templatization is something nice that we are eventually needing for the future, thanks for taking care of this.
Also code cleanup is something that is very welcome, as it removes stratifications of code.
I see also some improved syntaxes here and there that are most welcome as well.

As the general usual comment, there is a lot of changes in a lot of files both CPU/GPU. Surely for the templatization this is unavoidable, but it would be better to have the two streams of changing somehow separated in many PRs.
This would facilitate reviews and collaborations from other people (e.g. I have some developments ongoing on the GPU vertexer and I'd rather not have to deal with conflicts caused by refactoring included in a PR supposedly targeting a development).

[f3sch]

I checked that after the templatization the performance remains (within stat. error the same):

[mconcas]

@fprino this change is essentially an upgrade in the compatibility for trackers with more than 7 layers (e.g. ALICE 3).
It is also a required base for implementing a better protection against memory spike, which @f3sch is preparing in #14326.
There should be no functional change in the algorithms in this PR.
Upon your approval I'm merging.