Fix thresholds to extreme values

[cjvanlissa]

Mplus does:

 IN THE OPTIMIZATION, ONE OR MORE LOGIT THRESHOLDS APPROACHED EXTREME VALUES
 OF -15.000 AND 15.000 AND WERE FIXED TO STABILIZE MODEL ESTIMATION.  THESE
 VALUES IMPLY PROBABILITIES OF 0 AND 1.  IN THE MODEL RESULTS SECTION, THESE
 PARAMETERS HAVE 0 STANDARD ERRORS AND 999 IN THE Z-SCORE AND P-VALUE COLUMNS.

OpenMx does not do this. Consider doing a first run, then fixing these parameters, then running again.