Add 'gdal raster overview add' and 'gdal raster overview delete'

[rouault]

Docs previews: https://gdal--11614.org.readthedocs.build/en/11614/programs/gdal_raster_overview_add.html and https://gdal--11614.org.readthedocs.build/en/11614/programs/gdal_raster_overview_delete.html

[coveralls]

coverage: 70.083% (+0.009%) from 70.074%
when pulling ac7f7a0 on rouault:gdal_raster_overview
into 196f855 on OSGeo:master.

[rouault]

@jratike80 @sgillies Thoughts ?

[jratike80]

Thoughts ?

• I have always been thinking that -ro in gdaladdo is a programmer's view, but for a user it means external overviews. Would it be dangerous to have gdal raster overview add --external instead, even it is not explicit - overviews can be external for non-tiff formats anyway.
• What is the meaning of --read-only with gdal raster overview delete? To delete the overviews if they are external, but not if they are internal, or?
• "Integral overview levels" feel somehow odd but my English is not so good that I could tell why.

[rouault]

@jratike80 Thanks for the feedbak. "--read-only" renamed to "--external" to both "add" and "remove". "Integral..." rephrased to "A list of overview levels to build. Each overview level must be an integer value greater or equal to 2."