[Feat] r.kappa: improve output and add User's and Producer's accuracies

[neteler]

r.kappa lacks:

• User's and Producer's accuracies
• Output in CSV
• sort output differently:
  • arrange user and producer accuracies and omission and commission around the error matrix instead of the summary columns
• add notes to "map1" and "map2" to explain what that means (more user friendly)

For a working solution, see https://github.com/OSGeo/grass-addons/blob/master/grass7/raster/r.confusionmatrix/r.confusionmatrix.py

[neteler]

Note: for GRASS GIS 8, consider to rename r.kappa to r.validation or the like (as per https://github.com/OSGeo/grass-addons/pull/109#issuecomment-599960448)

[marisn]

r.kappa now supports JSON output. I'd guess CSV output is not required any more. See: #2666