Tools to diagnose interface condition enforcement

As a user, I would like to be able to export, on an interface (ie. codim 1 submesh), the following quantities:

$c_1/K_{S_1}$, $c_2/K_{S_2}$, and the residual $c_1/K_{S_1} - c_2/K_{S_2}$
(and equivalent for Sievert/Henry interfaces)

These space-dependent quantities would then be exported to VTX for visualisation.

Alternatively, we should also be able to export the integral of the residual as a derived quantity: $\int_\Gamma \left( c_1/K_{S_1} - c_2/K_{S_2} \right) dS$

Steps to achieve this:

- make a codim 1 submesh of the interface
- make a function space (CG1?)
- create `dolfinx.fem.Function` object each quantity
- create `dolfinx.fem.Expression` (⚠️ can this work if https://github.com/FEniCS/dolfinx/issues/3207 is not resolved?)
- interpolate expression in appropriate `Function` object in the iteration loop + write to file