Changing conda selectors syntax

[erikvansebille]

Changing the syntax for the conda [unix] selector (for MPI to install only on linux and osx), because sel(unix) was not widely supported and sometimes led to install problems

[VeckoTheGecko]

Just wondering about the motivation for 9bd78f0. From my understanding the mamba project is a much more performant version of the conda project, and operates as a drop in replacement.

[erikvansebille]

Just wondering about the motivation for 9bd78f0. From my understanding the mamba project is a much more performant version of the conda project, and operates as a drop in replacement.

I was trying to get the original sel(unix) commands to work on conda too (because I hoped that would work with the new libmamba solver). It's needed to make the the instructions for our local system at UU work, as well as make the installation instructions for developers simpler.

As I couldn't get that sel(unix) to work, I tried to revert back to the # [unix] preprocessing selectors (see here and therefore (temporarily) reverted back to conda instead of mamba. Nevertheless, that didn't seem to work in environment files either?

Anyways, I agree this PR is a bit of a mess, but trying to get installation instructions as simple as possible for conda. Any ideas how?

[VeckoTheGecko]

Looks like selectors aren't supported in environment.yml files (only meta.yml) conda/conda#8089. Not sure what the best solution to this is unfortunately. I guess we migrated away from maintaining 3 different environment.yml files?