Don't hard-code numeric conversion factor for mD->m**2

This commit replaces the hard-coded conversion factor from mD to
m**2 (metres squared) with an expression based on the Units.hpp
facility of module opm-core.
Request version information for Dune ISTL

The availability of Dune ISTL's FastAMG is enconded in opm-porsol's
class IncompFlowSolverHybrid<> using version checks on the ISTL
component.  Put the required information into "config.h" to
facilitate those checks.
List all available choices for "linsolver_type"

This commit updates the list of available linear solver types to
reflect the reality of the flow solver (IncompFlowSolverHybrid<>).  In
particular we record the existence of the "FAST_AMG/CG" alternative.
Use Dune ISTL's FastAMG preconditioner

This commit switches the linear solver used in the benchmark to the

    GeneralizedPCGSolver<> solver/FastAMG<> preconditioner

combination encoded in "linearsolver_type=3".  When available, this
reduces the computational cost (execution time) of a benchmark run
with a factor of about two compared to Dune 2.2.x's CGSolver<>/AMG<>
combination on the "small" case.  Strangely enough, the latter
combination *appears* to be about a factor of two *slower* in 2.3.x
than in 2.2.x, so this is altogether a run-time reduction of a
factor of about four compared to the original linearsolver_type=1 in
Dune 2.3.x.

The solver selection in IncompFlowSolverHybrid<> falls back to the
original CGSolver<>/AMG<> when GeneralizedPCGSolver<>/FastAMG<> is
not available (e.g., in Dune 2.2.x) so there's no adverse effect
from enabling linearsolver_type=3 unconditionally.
Merge pull request #46 from bska/enable-fastamg

Enable Linear Solver Based on GeneralizedPCGsolver and FastAMG