Add 3D reprojection for `DEM` objects and adjust documentation on use cases

[rhugonnet]

Since 3D reprojection through Rasterio is solved: rasterio/rasterio#2433 (comment)

We need to explain the different use cases on the "Vertical referencing" page:

• Same horizontal reference, different vertical reference: Use to_vcrs().
• Different horizontal and vertical reference at once: Use reproject().

For reproject(), we need:

• To force the use of APPLY_VERTICAL_SHIFT=TRUE in reproject() through Rasterio (might need a change in GeoUtils to support passing this argument),
• Raise error/warning if the PROJ vertical shift grid doesn't exist (GDAL/PROJ typically fails silently).

We also need to solve related inconsistencies from the initial implementation: #370, #628