**** NOT YET IMPLEMENTED ****
histeq_scale(z[, top= top_value][, cmin= cmin][, cmax= cmax])
histeq_scale returns a byte-scaled version of the array z having the property that each byte
occurs with equal frequency (z is histogram equalized). The result bytes range from 0 to
top_value, which defaults to one less than the size of the current palette (or 255 if no pli, plf, or
palette command has yet been issued).
If non-nil cmin and/or cmax is supplied, values of z beyond these cutoffs are not included in the frequency counts.