Fixed PETSc fieldsplit solver

The implementation of the PETSc field split solver was not correct and this has been corrected now. Also use hypre's BoomerAMG as a preconditioner for the pressure system instead of CG.
Importing sys twice is not necessary

Removed the second import statement
Print PyOP2 timing summary

By default, the PyOP2 timing summary is printed out at the end, this avoids having to remember setting the PYOP2_PRINT_SUMMARY environment variable.
Allow selecting solvers at runtime

Added now options use_petscsplitsolver and use_matrixfreesolver to the parameter file. Only they are set the PETSc/matrixfree solver are used.
Merge branch 'petsc_split_preconditioner'

This branch adds/fixes functionality for running the PETSc split solver.