Add r.usler algorithm

Author: Médéric RIBREUX

python/plugins/processing/algs/grass7/description/r.usler.txt

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+r.usler
+Computes USLE R factor, Rainfall erosivity index.
+Raster (r.*)
+ParameterRaster|input|Name of annual precipitation raster map [mm/year]|False
+ParameterSelection|method|Name of USLE R equation|roose;morgan;foster;elswaify|0
+OutputRaster|output|USLE R Raster