ref: "Multidimensional Scaling", Seminar Press, 1972, 1:105-155
ref: "Similarity and Choice", Hans Huber, 1980, pp. 234-289
see: prefmap.f, prefmap2.f, prefmap3b
prefmap3 is recommended over prefmap.f and prefmap2.f for most uses.
However, prefmap2.f can do some useful things prefmap3 cannot
prefmap3a (DOS version) has dynamic memory allocation which is more flexible
prefmap3b (DOS only) has static allocation which is simpler to use