Fix and implement modification indices for multiple group models

[mhunter1]

There is code that supposedly does modification indices for multiple group models (really just models that have submodels), but it expresses doubt about its own existence. See below from R/MxMI.R.

        # not yet tested
        if(length(model$submodels) > 0){
                for(asubmodel in names(model$submodels)){
                        ret <- c(ret, mxMI(asubmodel)) #probably won't work.
                }
        }

Unsurprisingly, upon a check, it did not work. The error message given by a multiple group model was

Error: object 'ret' not found

There is no previous reference to ret in the mxMI() function definition.

A basic test would use a multiple group behavior genetics model (perhaps with a "sideways" model reference). An alternative check would be a multisubject state space model. One could also try this with Rampart multilevel.