ll and se.approx, which calculates
the log-likelihood (and posterior mean and variance of the random effects)
and approximate standard errors, respectively. When either of these arguments
are TRUE, the function does not return the maaximizer from the MCEM
algorithm iteration, and instead reports the called for post-fit statistics.
stepEM(theta, l, t, z, nMC, verbose, gammaOpt, postRE, se.approx)