Outsource Cython backend

[MuellerSeb]

Now that we have GSTools-Cython, we can remove the cython code from GSTools.

[LSchueler]

It's really cool to see how fast GSTools now "build" with GSTools-Cython already built. Thank you this PR and for the new Cython repo!!

Two things to think about.

• Now that I read the installation guide again, I think that the part about building Cython with parallel support sounds pretty complicated, which it really isn't. Maybe first provide the code block and then the explanation for those who are really interested?

• Because the Rust code runs parallel without any extra installation steps when using pip, maybe that installation guide should come before the Cython part in order to keep it as simple as possible for the users? And maybe we should tone the text down a bit. It's probably not going to completely replace the Cython code anytime soon ;-)

But apart from that, I don't see anything to complain about, so LGTM ;-)

[MuellerSeb]

I would keep gstools-core on second place in docs, since there is no conda package for gstools-core. ATM I have no experience with putting a rust package on conda-forge. But I think it's no rocket science.

[MuellerSeb]

LGTM 🎉