FIML implementation

[jhorzek]

Hi Ross,
I just saw that you did some work on the FIML implementation in regsem. I've been programming a FIML implementation in a different package quite recently and thought that I might also contribute what I learned there to regsem. I've changed the fiml_calc4 function slightly. However, my implementation is quite rudimentary. Basically, FIML works but the fit indices and everything else that regsem computes after fitting the model might be wrong. It's currently all in the FIML branch of my regsem-fork (https://github.com/jhorzek/regsem/tree/FIML). I don't want to create a merge request to your main branch yet; the FIML implementation might break some things that I didn't think of. If you create a separate branch for development, I could contribute it there.
Best,
Jannik