Update covariates when model structure changes

[antoniaih]

Covariates need to be updated when the model structure is changed. If the user put a covariate on CL and then changes the model to Michaelis Menten elimination there is no CL parameter anymore and the program will crash. Covariates should be updated similarly to rvs in _update_rvs_from_model.

[antoniaih]

Fixed in 001497f

[rikardn]

This is a good point. I'll keep this open so we can double check how this works.