Fixes r.horizon - ticket #6760

GRASS r.horizon was not producing any output because it was missing the necessary field. It was anyway needed to remove the "horizonstep" parameter because using this GRASS will create >1 output and as far as I know this is not yet supported in processing. Even when creating just one output the module adds a "_0" suffix to it, so it was needed to patch GrassAlgorithm.py. I add also a "r.horizon.height" module to support the module 2nd way to work, that produces an output just in the GRASS log.