Make GDAL dependency optionnal and raise error in mesh_crue10_run.py if not install

[nicogodet]

GDAL can be very painfull to install (especially on windows without conda...).

As mesh_crue10_run.py is the only script that depends on GDAL when a specific argument is provided, I suggest to remove GDAL from requierements.txt and raise an error if args.infile_dem is provided and GDAL is not available.

If this is OK, I can create a PR.

[lucduron]

Ok if you want, although it should also be a dependency of PyTelTools (for a single script also).
I would keep it in .github/workflows/pythonpackage.yml.

I am also aware that import gdal has been replaced by from osgeo import gdal (see
https://github.com/OSGeo/gdal/blob/v3.2.1/gdal/NEWS), it should be updated.

On Windows, you can consider WinPython, it includes GDAL.