Make gstools-core the default and gstools-cython optional

[LSchueler]

This includes updating the installation instructions.

[LSchueler]

We now have a few Python wrapper functions, which don't actually do anything anymore. However, they at least show the default arguments in GSTools' code. Otherwise, you would have to check the gstools-core repo...
I'm not sure if that's worth it to keep the wrapper functions.

[MuellerSeb]

I think we can remove the wrapper functions. Defaults are documented in the high-level routines calling these.
Other this looks good to me.

Beside that we now need to get gstools_core onto conda-forge.

[LSchueler]

Done.
One thing still to decide: Shall pip install gstools[rust] throw an error or should we keep it for a while in case someone has an elaborate and automatized installation script?

We need gstools_core on conda forge before publishing a new version. So, merging this PR should be fine, as long as we keep that in mind.