 DISTS = PAIRDIST(X,Y), where X is an M x D matrix and Y is N x D,
   returns an M x N matrix DISTS such that DISTS(i,j) is the Euclidean
   distance between X(i,:) and Y(j,:).  X and Y must be real-valued.

   The algorithm used by default utilizes the expansion
                    (x-y)'(x-y) == x'x + y'y - 2x'y,
   because the right hand side uses fewer floating point operations.
   However, round-off error with this technique may produce small but
   non-zero (even negative) values when x == y.

   DIST