Resolve issue #15. mxRefModels: Doesn't handle models where data has only one column
Resolve issue #15. mxRefModels: Doesn't handle models where data has only one
column

As mentioned in the issue, it was just a needed drop=FALSE.

mhunter1/OpenMx,mhunter1/OpenMx,mhunter1/OpenMx,jpritikin/OpenMx,jpritikin/OpenMx,jpritikin/OpenMx,jpritikin/OpenMx,jpritikin/OpenMx,jpritikin/OpenMx,mhunter1/OpenMx,mhunter1/OpenMx,mhunter1/OpenMx