- Remove code for 'unwrapped' vectors

- provision for different MPI communicators

- solver class now selectable as 'petsc' instead of 'petscSolver'
  and uses 'petsc { .. }' dictionary instead of 'petscCoeffs { .. }'

- accept preconditioner from petsc dictionary or from parent.
  This makes the syntax looks a bit more natural.

- report PETSc solver name in performance information

- make reporting of solver settings option (verbose, non-verbose)

- avoid multiple initializations of PETSc

- provide hint about setting LD_LIBRARY_PATH

- although we already test for consistent sizes in the have_petsc
  check, add some extra safety here. This may also be useful in the
  future if using a mixed-precision solution.