Suppress external and warnings

This mirrors the PR in opm-autodiff, in that it contains the new UseWarnings.cmake version and guards dune and Eigen includes to avoid warnings from those sources. This PR also fixes some warnings in this module itself, and a bug in ImpesTPFAAD.cpp.

Some things that should be discussed and resolve before this is merged:
 - I have insufficient git skills to make the build system modification here have the same commits as in opm-core. Especially, it should credit @akva2 as originator. May be less of an issue when we at some point stop the build system duplication.
 - Currently, inclusion of CpGrid.hpp (from dune-cornerpoint) is warning-guarded. We may want to move the "outer fence" to that module, so that including its headers is not a problem.