PartDesign/ViewProviderDatum: minor enhancements

- added defaultSize constant
 - fixed typo in margin{g,}Factor function name

src/Mod/PartDesign/Gui/ViewProviderDatumLine.cpp

@@ -87,7 +87,7 @@ void ViewProviderDatumLine::setExtents (Base::BoundBox3d bbox) {
     // Add origin of the line to the box if it's not
     bbox.Add ( Base::Vector3d (0, 0, 0) );
 
-    double marging = bbox.LengthZ () * margingFactor ();
+    double marging = bbox.LengthZ () * marginFactor ();
 
     // Display the line
     pCoords->point.setNum (2);