Alti diff slope norm and angular diff

[adebardo]

Context

The goal of this ticket is to implement the equivalent of dem_processing from demcompare. These are altitude difference visualizations.
Thus, the following need to be implemented:

• AltiDiffSlopeNorm
• AngularDiff

Code

We propose implementing these two processes directly in the DEM class, with the creation of two methods:

def alti_diff_slope_norm() and def angular_diff().

Example Usage:

dem1 = xdem.DEM("path1")
dem2 = xdem.DEM("path2")

diff = dem1 - dem2
slope_norm = diff.alti_diff_slope_norm()

The idea is to take inspiration from what is done for terrain attributes.

For alti_diff_slope_norm:

• Adapt this class and its [code](https://github.com/CNES/demcompare/blob/master/demcompare/dem_processing/dem_processing_methods.py#L146).

For angular_diff:

• Adapt this class and its [code](https://github.com/CNES/demcompare/blob/master/demcompare/dem_processing/dem_processing_methods.py#L324).

Additionally:

• Add logging to make it clear to the user that this function only makes sense if the DEM is an altitude difference.

For Tests

Take inspiration from what was done in demcompare.

Documentation

Update the documentation.