This routine is a utility routine that does not check the
input arguments for proper dimensions and type.
Details
simmnl will simulate two uniformly distributed X vars and add intercepts.
References
For further discussion, see Bayesian Statistics and Marketing
by Allenby, McCulloch, and Rossi.
http://gsbwww.uchicago.edu/fac/peter.rossi/research/bsm.html