clean up PETScPreconditioner

The interface PETScPreconditioner.* provides an overwhelming assortment of detailed options, e.g.,

prec.parameters['hypre']['BoomerAMG']['relax_type_coarse'] = ...

PETSc itself features many more options; to make those generally available, the PETScOptions mechanism was added a while ago. The above can now be represented by

PETScOptions.set('pc_hypre_boomeramg_relax_type_coarse', ...)

and the code for adding that explicit option to the parameter set in PETScPreconditioner is redundant. The same is true for 90% of what resides in PETScPreconditioner.*.

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