Make xdem convertible to binder

[adehecq]

It would be great to create a binder from xdem repository directly. To do that, I see two issues:

1. Once the Binder created, starting a console and running import xdem raises "ModuleNotFoundError: No module named 'xdem'". Something must be changed to xdem's configuration probably
2. All our examples are python scripts, that are not so nice to work with on Jupyter Lab. If jupytext is added as a dependency, the jupyterlab will be able to open any script as a notebook. With jupytext, it works but the notebooks look quite ugly (see screenshot below), so we need to improve that.

To reproduce:

1. Go https://mybinder.org/ and use URL GlacioHack/xdem to start the Binder.
2. Install jupytext with conda install -c conda-forge jupytext, then start jupyter notebook and open any of xdem's example scripts

[fmaussion]

Where is your binder configuration? Usually it would be in a binder folder at the repo root level

[adehecq]

We don't have one 🙃 I thought all that was needed was the setup.py.
I don't find any in OGGM. Do you have an example of a configuration somewhere? Because I tested running mybinder with OGGM/oggm and import oggm works...

[fmaussion]

Because I tested running mybinder with OGGM/oggm and import oggm works...

No kidding... Times have changed!

OK so OGGM uses quite a few shenanigans to share the same environment across many different binder links, which I don't recommend here. for other reps we usually have a binder folder like here: https://github.com/OGGM/glacier_simulator/tree/master/binder

[adehecq]

Adding the binder folder helps creating a new environment, but somehow opencv does not install correctly. I get the same error as here. I'll try the same fix.