add grass7 r.cost module with no need for stop points input

Author: gioman

python/plugins/processing/grass7/description/r.cost.full.txt

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+r.cost
+r.cost.full - Creates a raster layer of cumulative cost of moving across a raster layer whose cell values represent cost.
+Raster (r.*)
+ParameterRaster|input|Unit cost layer|False
+ParameterVector|start_points|Start points|0|False
+ParameterBoolean|-k|Use the 'Knight's move'; slower, but more accurate|False
+ParameterBoolean|-n|Keep null values in output raster layer|False
+OutputRaster|output|Cumulative cost