To-do before v0.1

[rhugonnet]

The things to do before releasing a 0.1 for xDEM are:

• Add wrappers for all features (terrain, coregistration) as DEM class methods #474 then update related documentation example to make them even more accessible,
• Describe GeoUtils-xDEM link in documentation #275 without forgetting to copy the Raster API to help users,
• Deal with all warnings in tests making UserWarning fail tests everywhere,
• Write more useful error message that propose solutions.

Coregistration improvements can be left for a next minor version, as well as elevation point clouds, etc.