Computes the density function for the Wilcoxon rank-sum distribution, using
exact values when both groups have fewer than 50 items and switching
to a normal approximation otherwise. It was originally written for
S-Plus, which still perversely insists that m and n must
be less than 50. The function was retained when the OOMPA library was
ported to R, since S-Plus keeps the actual rank-sum but R centers the
distribution at zero. This function encapsulated the difference, allowing
everything else to continue to work as it had worked previously.