Optimization w/o EM algo, using one-shot fsolve

Removed iterative procedure for EM algorithm

Instead solve first order condition directly. Based on equation (2.22) in MacLachlan and Peel book.

To make problem simpler, presume knowledge of prtype, i.e. type probability does not need to be
estimated here.

remaining problem: results in the simple univariate probit estimator b/c it can simply set the
estimate for mu equal to zero which is consistent with foc.