Allow accurate measurement close to DSM resolution

Easy way to do it otherwise it requires to parse r.info output

plugins/measure/calc_volume.py

@@ -34,7 +34,10 @@ def main():
     Module("v.buffer", input="polygon_area", s=True, type="area", output="region", distance=1, minordistance=1, overwrite=True)
     Module("r.external", input=opts['dsm_file'], output="dsm", overwrite=True)
 
-    # Set Grass region to vector bbox and resolution to DSM
+    # Set Grass region and resolution to DSM
+    Module("g.region", raster="dsm") 
+    
+    # Set Grass region to vector bbox
     Module("g.region", vector="region")
 
     # Create a mask to speed up computation