ENH: changed the treatment of fvOptions in primal solvers

fvOptions are no longer a member of incompressiblePrimalSolver but are
looked up from the registry in each iteration of each primal solver.
This means that the main system/fvOptions dictionary is read by ALL
instances of the primal solvers and the latter no longer have their
own fvOptions dict in optimisationDict. This is safe since each fvOption
is applied to a specific field and in case of many primal solvers, the
primal fields are named differently for each of them.

In addition, simple is now split in preLoop, loop and postLoop phase.
Furthermore, each SIMPLE iteration is broken down to
a preIter, mainIter and postIter phase, to allow for different behaviour
by derived classes.