CellSetExplicit::AddCell new variation

Right now, AddCell requires a Vec for the cell's point IDs, but people would like to pass in e.g. a int*. Given that we only need that to support operator[], we could just change that last parameter to a fully templated type, e.g.

change:

template <vtkm::IdComponent ItemTupleLength>
void AddCell(vtkm::IdComponent cellType,
           int numVertices,
           const vtkm::Vec<vtkm::Id,ItemTupleLength> &ids)

... to:

template <typename IndexableType>
void AddCell(vtkm::IdComponent cellType,
            int numVertices,
            const IndexableType &ids)

(or similar)