Introduced a simple fluid which has no problem with strange black oil behavior.
Intended for testing, but for now it is used in SaturationPropsFromDeck.
This is a change from taking a vector containing the mapping to
deck-consistent logical cartesian indices. The mapping is contained
in the UnstructuredGrid::global_cell member, and may be null. The
change therefore saves the overhead of constructing a vector as a
copy of the data in the grid or (if null) as an identity mapping.