PBiCG: Suggest changing to the more robust PBiCGStab solver

if convergence is not achieved within the maximum number of iterations.

Sometimes, particularly running in parallel, PBiCG fails to converge or diverges
without warning or obvious cause leaving a solution field containing significant
errors which can cause divergence of the application.  PBiCGStab is more robust
and does not suffer from the problems encountered with PBiCG.