BUG: optimisation convergence criteria misbehave w/o a line search (fixes #3268)

When using the (optional) convergence criteria for the optimisation
loop, adherence to them is checked in the postUpdate phase of the update
of the design variables. If a line search is used, this means that the
flow equations have been solved anew using the new design variables and
the objective reduction check makes sense. If, however, no line search
is used, the objective reduction check happens immediately after the
update of the design variables and before re-solving the flow equations,
giving a reduction in the objective function and terminating the
optimisation loop prematurely.

Solved by calling the convergence check directly after the solution of
the flow equations, in case of a fixedStepUpdate