To-do before `v0.1`

[rhugonnet]

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

• [x] #474 then update related documentation example to make them even more accessible,
• [x] #275 without forgetting to copy the Raster API to help users,
• [x] Deal with all warnings in tests making UserWarning fail tests everywhere,
• [x] 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.