Allow for observation-level simulation and new data in simulate_gls and simulate_gnls

[femiguez]

I will target these two functions first before I try to tackle lme and nlme

[femiguez]

This is largely implemented. Now testing... Testing reveals that handling groups is fairly slow. Need to make this more efficient. Update: major speed up of handling of groups in variance structure. Next, I should incorporate the ConstPower model.

[femiguez]

Done