Allow group variable to be passed as variable name when using formula

This is a small PR to allow the group variable to be passed to MixedLM.from_formula as either a variable name, or as an array-like object containing the actual group values.  Currently the group values must be passed as an array.
how long do these take to fit?

since it's not important here, and in the other fit, to check convergence, we could add `start_params = rslt2.params * 0.9` for example.