Add AMG option for CPR preconditioner

This adds the option to use an AMG preconditioner for solving the elliptic part in the CPR preconditioner.

It has been tested to compile with Dune 2.2 and 2.3. The option "cpr_use_amg" defaults to false, so with no input change, existing behaviour is preserved. With the option set to true, It has been tested on SPE9 and a reduced Norne case. On SPE9 the results are not exactly the same, but equivalent to the old ones, but it was a little slower. On Norne, it is significantly faster on my machine (34 seconds to 24 seconds for the first time step with 5 nonlinear iterations).