Implement strongly enforced (internal) Dirichlet constraints for cell-centered models

This is a follow up of #743 (closed). The cell-centered implementation of internal Dirichlet contrainst was incorrectly introduced with !1664 (merged). The wrong implementation is removed in !1725 (merged) and replaced by throwing exception in the case the feature is enabled. This is a reminder that a correct implementation is still missing.

This is a feature request:

• Implement strongly enforced Dirichlet constraints for any user-chosen cell, using the interface implemented in !1664 (merged).