It turns out that the XML reader calls GetNumberOfPoints() during RequestInformation().
Therefore, this cannot be obtained from the output data (because it is not populated).
Previously, the hyperoctree reader was returning 0. With my last change, it started
crashing because output is now NULL. Changed to return 0 if output is NULL. This
is not the right behaviour because meta-data gets the wrong number of points.
However, there is no easy way to compute the number of points/cells without actually
reading the data so it stays.