Handling DEM with no overlapping pixel

[aburie-cs]

For now, only DEM cut in one degree TILE WITH ONE OVERLAPPING PIXEL are handled by Sen2VM.

This ticket is to track the enhancement of Sen2VM to b able to handle DEM without overlaps in the 1 square degree tiles.

• Update the code to handle DEM with no overlapping pixels
• Update tests with one DEM with no overlapping pixel (COG on AWS ?)
• Update documentation

[aburie-cs]

For this, we need an update of to new official RUGGED 4.0.1.
This RUGGED is included in SXGEO 0.3.1.