This commit passes the run's notion of its active phases, an object
of type Opm::Phases, through to the initialisation layer for the
saturation functions' scaling properties. In particular, this
allows us to discriminate between the phases and to not index into
tables or properties that would not be appropriate (e.g., maximum
gas saturation (SGU) in a simulation run without active gas).
Moreover, we now have enough information to know to look for SOF2 in
two-phase run using family II saturation function keywords. These
changes are necessary in order to extend Flow's support for the
FILLEPS output request to two-phase runs.
