Function tangent_plane_at 

pub fn tangent_plane_at(
    patch: &BoundedPlane,
    CartesianCoordinates: CartesianCoordinates,
) -> BoundedPlane

Plane to cut one element with, when the face it crosses is curved.

get_interface_plane gives the plane tangent at the middle of the compartment, which drifts away from the patch as theta moves:

      tangent at theta_c
     ------+------            an element sitting here never reaches the tangent plane,
      __--- ---__             it contributes no area at all
    _-     |     -_  <- patch
   /       |       \
          axis

Taking the tangent at the angular position of the element keeps the error down to the curvature over one element instead of over one compartment.