Add const version

Author: dschwoerer

include/bout/field2d.hxx

@@ -282,6 +282,7 @@ public:
   int size() const override { return nx * ny; }
 
   Field2D& asField3DParallel() { return *this; }
+  const Field2D& asField3DParallel() const { return *this; }
 
 private:
   /// Internal data array. Handles allocation/freeing of memory