Bundle the boost git repo instead of using the .tar.gz archiv

[lenaploetzke]

Motivation / Current Behaviour

We currently bundle only a minimal extract of boost library as an archive that contains only the libraries of boost that we use. E.g. in PR #929 we need additional functionality from boost. The resulting bundle of boost is too big to include it in the repository.

Enhancement description

The preferred solution is to bundle the git repo of boost instead of using the .tar.gz archive. A disadvantage is that the build step of the project takes a while because boost is downloaded.

Talked to @mknaranja, @reneSchm and @annawendler and we have agreed that the current preferred solution is to always download boost by default and no longer offer a minimal version. This has the advantage that the minimal version does not have to be updated when changes are made. The download time is acceptable. It should also be possible to use a locally installed boost version instead of downloading boost.
In the future, we want to offer the option of not downloading boost, whereby only limited functionality can be used. See Issue #992 .

Additional context

No response

Checklist

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[xsaschako]

I already tried this. As far as i know, there is no elegant way to do this, apart from bundling it.
Do you have an idea in mind?
There currently is minimal CMake support for boost for this but to use it i think it is not ready yet. Last time i looked into it was 1/2 year ago.

[lenaploetzke]

Yes, I have a few ideas and some things have already worked out. I've also talked to @dabele. Maybe I'm missing something that's causing the whole thing to fail. I'll try a few things. Will keep you updated.