fix for sextante/convexhull tool based on field

the tool does not work if option "Create convex hulls based on field" is used
to due of missing some methods. This patch fixes the issue.

python/plugins/sextante/ftools/ConvexHull.py

@@ -184,6 +184,28 @@ def processAlgorithm(self, progress):
         if not FEATURE_EXCEPT:
             SextanteLog.addToLog(SextanteLog.LOG_WARNING, "Feature exception while computing convex hull")
 
+    def simpleMeasure(self, inGeom ):
+        measure = QgsDistanceArea()
+        attr1 = measure.measure(inGeom)
+        if inGeom.type() == QGis.Polygon:
+          attr2 = self.perimMeasure( inGeom, measure )
+        else:
+          attr2 = attr1
+        return ( attr1, attr2 )
+
+    def perimMeasure(self, inGeom, measure ):
+        value = 0.00
+        if inGeom.isMultipart():
+          poly = inGeom.asMultiPolygon()
+          for k in poly:
+            for j in k:
+              value = value + measure.measureLine( j )
+        else:
+          poly = inGeom.asPolygon()
+          for k in poly:
+            value = value + measure.measureLine( k )
+        return value
+
     def defineCharacteristics(self):
         self.name = "Convex hull"
         self.group = "Geoprocessing tools"