updated doc for new Python interface via cmake

[rafaelab]

Minor changes to the documentation to reflect the recent updates.

[lukasmerten]

I remember that is was quite tricky to install crpropa on some Mac configuration, so I would suggest to keep the longer explanation for a little longer.

But maybe @reichherzerp, @sophieaerdker, or @LeanderSchlegel can comment on that.

[rafaelab]

I remember that is was quite tricky to install crpropa on some Mac configuration, so I would suggest to keep the longer explanation for a little longer.

I tested it on my machine, @lukasmerten, and apparently things work out of the box now. I decided to remove it to avoid confusion. Let's see if the same is true for the others. But sure, maybe we can keep it as legacy information.
Btw, our OSX tests are passing without any of these flags.

[rafaelab]

I adjusted the previous version now, @lukasmerten .
I think the reason for that complicated installation had something to do with AppleClang/LLVM and OpenMP, that is why all the flags were required. I think the newer versions of the compilers handle OpenMP much better.

[JulienDoerner]

Hey @rafaelab,
this PR looks good for me. I would have only one minor question. We state "testet for > Python 3.0 and > SWIG 4.0.2", but SWIG itself requires at least Python 3.2. So my question is, which version of Python did we realy tested? The oldest version I ever used was 3.7.

[rafaelab]

Hey @rafaelab, this PR looks good for me. I would have only one minor question. We state "testet for > Python 3.0 and > SWIG 4.0.2", but SWIG itself requires at least Python 3.2. So my question is, which version of Python did we realy tested? The oldest version I ever used was 3.7.

Good point, @JulienDoerner. I said 3.0 just because we don't support Python 2 anymore.
I didn't actually test versions prior to 3.7.
What is your suggestion? Should we say 3.2, or 3.7?

[lukasmerten]

@rafaelab: Just put whatever is the oldest python version that is used in one of the actions. I guess that is python 3.8 in the ubuntu-20 runner.