dS integrals are not part of the integration kernels

Hello,

As raised in [this post](https://fenicsproject.discourse.group/t/periodic-boundary-condition-with-dg-and-mpc/12957/6), `dolfinx_mpc` can not be used with DG discretization because "dS integrals are not implemented as part of the integration kernels" ([link to the code](https://github.com/jorgensd/dolfinx_mpc/blob/main/cpp/assemble_matrix.cpp#L608-L626)). 

Cheers,

Lucas