[Feature Request] Optional spontaneous curvature field in `MeanCurvatureSq`

[joshichaitanya3]

In the current implementation of the MeanCurvatureSq functional, we cannot set a value of spontaneous curvature, which would be needed to model closed shells, for example.

[softmattertheory]

This is an important suggestion, @joshichaitanya3. We can work on it together; I think it needs oriented meshes to get the sign of the curvature correct, right?