Multigroup SEM constructor

[brandmaier]

I suggest to add a shortcut multiple group SEM constructor that looks like this:

model = MultipleGroupsSem(
    specification = partable
    data = dat,
    by = column
)

which creates an SEMEnsemble with groups corresponding to the number of unique entries in column column in dataset dat.

[Maximilian-Stefan-Ernst]

Thanks for the request, the constructor will be added to the next release.