QBXLayerPotentialSource.copy(): Fix _refined_for_global_qbx
to have a default argument of _not_provided (closes #59 (closed) on gitlab).
Without this fix, the copied layer potential source thinks it is not refined.
to have a default argument of _not_provided (closes #59 (closed) on gitlab).
Without this fix, the copied layer potential source thinks it is not refined.