0.6.3

• Use norm of right-hand side instead of initial error for scaling residuals in
  iterative solvers. This yields consistent results when starting with a decent
  initial approximation.
• Provide an HPX backend.
• Accept near null-space vectors for aggregation-based coarsening.
• Added SPAI(1) smoother (may work better than SPAI(0), but is extremely slow
  to setup for larger problems).
• Iterative solvers now provide apply() methods, so that they may be used as
  preconditioners themselves.
• Various fixes and improvements.