namely BlackoilStateToFluidState which takes a BlackoilState object
and exposes it as a opm-material like fluid state object. Similar for
ExplicitArraysFluidState, which takes raw arrays.
since fluid states are a local API, the index of the cell to be used
for these two classes must be set externally. The advantage of this
concept is that it is possible to make "saturation functions" which
not only depend on saturations but also on arbitrary other quanties
(like temperature or phase composition) without having to change the
API of the "saturation" functions.
