Fix build failure

[rhugonnet]

• Refactored a missing spatial_tools.nmad into spatialstats.nmad
• Removed the NMAD section reference in the documentation (was deleted along with Mean, Median, etc when writing the Robust statistics page, refers to Wikipedia instead)
• Forced data type to int for calls using rasterio.xy.transform that triggered error after rasterio 1.2.7 release yesterday, and opened issue in rasterio: rio.transform.xy does not accept all float and int data types when passing a single row/col rasterio/rasterio#2283
• Skipped a test of blockwise co-registration that also appeared, opened the xdem issue test_blockwise_coreg_large_gaps fails with all NaN assertion in coreg.apply_matrix #211

[rhugonnet]

The 1.2.8 release of rasterio fixes the data types issue, I reverted back the changes for those.
Other opened issues stay the same.
This PR is ready to be merged!

[erikmannerfelt]

Good job!