Add parameters controlling amg usage in cpr preconditioner.

New parameters are:
 - cpr_use_amg      (default false) if true, use AMG preconditioner for elliptic part
 - cpr_use_bicgstab (default true)  if true, use BiCGStab (else use CG) for elliptic part
Merge pull request #207 from atgeirr/params-for-amg

Add parameters controlling amg usage in cpr preconditioner.