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.

This supersedes (parts of) OPM/opm-core#283