The CellSet classes all exclusively work in the control environment.
However, CUDA likes to add __device__ to constructors, destructors, and
assignment operators it automatically adds. This in turn causes warnings
about the __device__ function using host-only classes (like
boost::shared_ptr). Solve this problem by adding explicit methods for
all of these.
When you create a CellSetPermutation you provide an array of the cell ids that
you want to iterate. This allows the user to do custom blanking of a data set,
or to do multi iteration over a set of cells.
