Add ismodelbased method.
Test for model-based chains in dic and logpdf.
Add state field to Model type.
Add setinits! for initialization vector.
Add mcmc restart method for Chains, and revise mcmc method for Model.
Update mcmc argument name to reflect code change.
Rename Model `state` field to `states`.
Remove unneeded getstate method.
Reset iteration number in setinits!.
Fix logic for burnin and thin.
Add documentation for restarting chains.