Attempting to integrate an interior surface integral

It's unclear to me how to integrate an interior surface, or if it's even possible. I attempted to grab faces using `serac::Domain::ofFaces` and then give this domain to a `serac::Functional` boundary integral, but I get this error- "Error: trying to evaluate a boundary integral over a non-boundary domain of integration"

Start by reproducing the error with serac::Domain::ofFaces and a serac::Functional boundary integral. Read the domain and boundary-integral entry points to determine how interior faces are represented and whether they are supported. Done means establishing the supported behavior and documenting or implementing a clear path for interior surface integrals.

Written by the indexing model from the issue text.