The value of _array_intensities.pixel_fast_bin_size  specifies
              the number of pixels that compose one element in the direction
              of the most rapidly varying array dimension.
              Typical values are 1, 2, 4 or 8.  When there is 1 pixel per
              array element in both directions, the value given for
              _array_intensities.pixel_binning_method  normally should be
              'none'.
              It is specified as a float to allow for binning algorithms that
              create array elements that are not integer multiples of the
              detector pixel size.