ENH: changes related to when the objective value is written

- Objective now inherits from localIOdictionary and writes the mean
objective value under the uniform folder, each time mesh.write() is
called. This is crucial for getting the correct old merit function value
if the simulation is continued from a previous state and lineSearch is
used.
- Objectives are now computed and written even if the corresponding
adjoint solver is inactive. This, among others, is also essential for
getting the correct old merit function value in case of continuation.
- Writing of the objective function (and its mean, if present) history
has now moved to updatePrimalBasedQuantities, instead of the preLoop
part of the adjoint solvers. This was decided to get the objective
values to files, even if the adjoint solver is inactive. Arguably, an
even better place to write the objective functions would be the postLoop
part of the primal solvers, however this might cause multiple writes of
the objective value for the inner iterations of lineSearch, if one is
used.