32 bit cell array converter fix

In a few files, the vtkCellArray of vtkUnstructuredGrid or vtkPolyData was set to 32 bits if there were less than 2^32 points. Since point ids are signed integers, we can only use 31 bits on a 32 bits array to store point ids. So now the conversion to 32 bits is done if there are less than 2^31 points.