Review code for exposed models; test recovery of simulated data

[jburos]

Not all of the models currently exposed in this library have been tested as robustly as others.

Also, some of the code may be out of date. This is a placeholder task to track progress reviewing code & updating the test cases to make sure we recover parameters for simulated values.

Models that need to be covered include:

1. weibull_survival_model
2. exp_survival_model
3. pem_survival_model (unstructured)
4. pem_survival_model_varying_coefs (unstructured)
5. pem_survival_model_timevarying
6. pem_survival_model_randomwalk
7. pem_survival_model_gamma