Add a set of points to the PointPairList from two arrays of type double.
If either array is null, then a set of ordinal values is automatically
generated in its place (see Ordinal.
If the arrays are of different size, then the larger array prevails and the
smaller array is padded with Missing values.
The zero-based ordinal index where the last point was added in the list,
or -1 if no points were added.
