Recall that the class that used to be called SinglePvtDead has
been renamed to SinglePvtDeadSpline. If 'props_use_spline' is true,
that class is used (this is the default), which makes a monotone
spline that is uniformly, densely sampled. The new class simply
uses linear interpolation in the input tables.
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.
