Provide way of shadowing only some properties

The current implementations of IncompPropertiesInterface are very
all-or-nothing. In some situations, you want to read rock and fluid
properties from an Eclipse file, but use analytical functions for
the unsaturated properties. Or you want to update properties based
on a marching filter.

This patch provides a way to mix various property objects, or to
"shadow" the properties with a raw array of data, so you don't have
to reimplement the entire interface just to make a small change.

tests/test_shadow.cpp

@@ -0,0 +1,32 @@
+/* Copyright (c) 2013 Uni Research AS.
+   This file is licensed under the GNU General Public License v3.0 or later. */
+#include <config.h>
+
+#if HAVE_DYNAMIC_BOOST_TEST
+#define BOOST_TEST_DYN_LINK
+#endif
+#define NVERBOSE // to suppress our messages when throwing
+
+#define BOOST_TEST_MODULE ShadowTest
+#include <boost/test/unit_test.hpp>
+
+#include <opm/core/utility/parameters/ParameterGroup.hpp>
+#include <opm/core/props/IncompPropertiesBasic.hpp>
+#include <opm/core/props/IncompPropertiesShadow.hpp>
+
+using namespace Opm;
+
+BOOST_AUTO_TEST_CASE(shadowPorosity)
+{
+    const double defaultPorosity = 1.0;
+    const double newPorosity = 0.5;
+
+    parameter::ParameterGroup param;
+    IncompPropertiesBasic basic (param, 2, 1);
+    IncompPropertiesShadow shadow (basic);
+    BOOST_CHECK_CLOSE (*(shadow.porosity()), defaultPorosity, 0.001);
+    shadow.usePorosity (&newPorosity);
+    BOOST_CHECK_CLOSE (*(shadow.porosity()), newPorosity, 0.001);
+    shadow.usePorosity (basic);
+    BOOST_CHECK_CLOSE (*(shadow.porosity()), defaultPorosity, 0.001);
+}