Make xdem compatible for Binder

[adehecq]

To address #296

This PR:

• Adds a binder repository
• In this folder, adds a postbuild file to run command pip install -e . to install xdem
• install "libgl1-mesa-glx" which is needed, otherwise import cv2 raises error "ImportError: libGL.so.1: cannot open shared object file: No such file or directory"
• install optional dependencies using extra-environment.yml -> used to install jupytext, for turning .py files into notebooks on the fly.
• apparently, if binder folder exists, BinderHub does not take into account the environment.yml file in the root directory, so I had to create a symlink to that in the binder folder. Don't know if that's a good idea, or if we should duplicate the file and add a test to check that it is updated at the same time as the main environment.yml.
• added pip as a dependency, otherwise BinderHub complains (without stopping) during build.

The binder can be tested at https://mybinder.org/v2/gh/adehecq/xdem/binder.

[adehecq]

Commit 5ab5f06 causes Python version 3.7.12 to be installed. Binder logs say it's because this is required by xdem... 🤔

[rhugonnet]

Looks good!
The mypy error in pre-commit and the pytest errors on RuntimeWarning for divide are now fixed in #304, so your checks should run fine once that PR is merged.

[rhugonnet]

Happy that my routine in test_environment was useful in the end, it detected that pip was added only in environment.yml and not dev-environment.yml! 😛

All tests passing after merging with main, looks ready to merge!