Robustify mxRefModels

When the Cholesky of the pairwise complete sample covariance fails, try just starting with the variances and means.

http://openmx.psyc.virginia.edu/thread/4079#comment-6338

Fixed with https://github.com/OpenMx/OpenMx/commit/92d7bbf0160f202bf909851cd514aca99ae0f4bf