Find many image statistics in a single pass through the PELs. Returns a
DOUBLEMASK of 6 columns by n+1 (where n is number of bands in image in) rows.
Columns are statistics, and are, in order: minimum, maximum, sum, sum of
squares, mean, standard deviation. Row 0 has statistics for all bands
together, row 1 has stats for band 1, and so on.