Merge pull request #4887 from NaturalGIS/grass_218_voronoi_skeleton

port GRASS7 v.voroni.skeleton to 2_18

Author: alexbruy

python/plugins/processing/algs/grass7/description/v.voronoi.skeleton.txt

@@ -0,0 +1,7 @@
+v.voronoi
+v.voronoi.skeleton - Extract skeletons for input areas.
+Vector (v.*)
+ParameterVector|input|Input polygon layer|2|False
+ParameterNumber|thin|Maximum dangle length of skeletons (-1 will extract the center line)|None|None|-1
+Hardcoded|-s
+OutputVector|output|Area skeleton