Add constant field types

[landinjm]

Add a constant field type for spatially varying but time invariant fields.

This way we don't have to update a field at every time step (just when we adapt the mesh with solution transfer)

[landinjm]

Here's an old commit that did that. Just have to port the code to the most recent version: https://github.com/prisms-center/phaseField/pull/203/commits/6195c3c07a5a708688c5b92255c10093706fe0f5