BUG: fan: bc value not updated. See #3211

The problem is present anywhere a field with constraint patch override (patchType XXX) gets calculated. In this case the RHS gets calculated with calculated and the constraint type so when this gets assigned to the LHS it is not up to date with the behaviour override.

• it will generally not be seen in solvers where the main variable is the one being solved for since it will call the correctBoundaryConditions which will fix all. It is only on those solvers where a derived variable is solved for (e.g. p_rgh instead of p).
• the override behaviour is only possible for fields that are read in from dictionary.