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

Without this PR, the AMG option is present in the preconditioner, but with no way to activate it.