Add r.uslek algorithm

Author: Médéric RIBREUX

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

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+r.uslek
+Computes USLE Soil Erodibility Factor (K).
+Raster (r.*)
+ParameterRaster|psand|Name of soil sand fraction raster map [0.0-1.0]|False
+ParameterRaster|pclay|Name of soil clay fraction raster map [0.0-1.0]|False
+ParameterRaster|psilt|Name of soil silt fraction raster map [0.0-1.0]|False
+ParameterRaster|pomat|Name of soil organic matter raster map [0.0-1.0]|False
+OutputRaster|output|USLE R Raster