Wrap all functionalities as DEM methods

[rhugonnet]

This PR:

• Adds all of xDEM's methods as wrappers in the DEM class, for instance DEM.slope(), DEM.coregister_3d(), DEM.estimate_uncertainty(),
• Adds tests to check these run correctly (simple comparison to outputs of module functions),
• Adds a copy_doc decorator in misc.py that allows to copy the docstring of terrain function descriptions in the DEM class without having to re-write it all,
• Adds to the documentation a new "Elevation data objects" section containing a "The digital elevation model" page that describes the DEM object (and soon an "Elevation point cloud", and "Xarray DEM accessor"),
• Adds a "Quick use" subsection on the pages "Coregistration" and "Terrain attributes" to mention the options of calling the functions from a DEM, or to use the module directly,
• Updates the "Spatial statistics" page into an "Uncertainty analysis" page, in preparation for Re-structure spatialstats.py #378,
• Updates the API of xDEM to mirror part of GeoUtils, or point towards it.

Resolves #474
Partially addresses #275

I will further improve the uncertainty analysis page in #378, and we can improve the one of the coregistration after our hackathon! Then we'll be close to a fully populated and homogeneous documentation for a v0.1! 😄

[rhugonnet]

Ready to merge, other documentation change will take place in future PRs.
Maybe the most efficient is to review the entire documentation at once it's over? By default it won't be visible for users until we release a new version (will be accessible in latest, but stable is the default one).

[rhugonnet]

Functions already tested, only wrappers. And for the documentation, let's review all at once before the first minor release, it'll be more practical!
Merging.