Add caching to CI environment setup

[rhugonnet]

Mirroring GlacioHack/geoutils#321 in GeoUtils, this PR enables the caching of conda environment using the methods of conda-incubator/setup-miniconda and github/actions/cache to avoid long running CI times, especially on Mac and Windows.

In this PR, the functions environment_yml_nopy and diff_environment_yml are fixed for a couple issues when passing pip dependencies, that wen't unnoticed because they weren't needed in GeoUtils. I will open a PR to fix those in GeoUtils as well.

In short

CI durations before:

• ubuntu: 15-20 min, with 3-8 min for environment setup.
• mac: 23-30 min, with 7-15 min for environment setup.
• windows: 23-30 min, with 7-15 min for environment setup.

CI durations after:

All OSs take 11-18 min, with less than 1 min for environment setup (when uncaching) on Linux and Mac 🥳!
On Windows, the uncaching can take longer (up to 7 min) due to the archive extraction issue mentioned in GlacioHack/geoutils#321, that should be solved by GitHub Actions eventually.

Details

See GlacioHack/geoutils#321

Resolves #308

[rhugonnet]

Ready to merge! 😄

[adehecq]

Looks complex... Nice work !