FIX: roll-back of the execution of function objects in optimisation

In steadyOptimisation mode, each time-step corresponds to an
optimisation cycle and is sub-cycled, to allow for iterating the flow
and adjoint equations. This sub-cycling does not allow the execution of
function objects. This was circumvented in 8947735b, by explicitly
calling the execution of the function objects in the simple solver
of adjointOptimisationFoam.

However, each sub-cycled iteration is a writeTime, if the current
optimisation cycle corresponds to a writeTime. This means that function
objects with a

writeControl write;

will be executed in each iteration of the flow equations, within this
specific optimisation cycle, leading to a lot of disc space and file
clutter, if the function object outputs fields (e.g. yPlus).

8947735b is partially rolled back, by protecting the call to the
execution of the function objects with a bool that defaults to false.