Add a warning when meanstructure=false and means are specified?

[brandmaier]

Currently, if I do

model = Sem(
           specification = partable,
           data          = data,
           meanstructure = false
       )

and the partable has a mean structure specified, this is just fine. I wonder whether this should deserve a warning message? Let's discuss.

[Maximilian-Stefan-Ernst]

Yes thats true, I will have to think about how to implement this warning.

Related: FIML needs a meanstructure, so I think we should warn if users don't specify one.