make all non-implementation headers includable without
preconditions. Also, this removes the GravityColumnSolver.hpp file,
because it tried to include a non-existing file and it was thus unused.
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.
