`AdvancedHMCExt` will use `evaluate!!` to rerun the model for every sample drawn, and it seems wasteful

[sunxd3]

See conversation #93 (comment)

[sunxd3]

We can add generated_variables as a Model field. And then have a customized AbstractMCMC.step to save the samples into the chain.

It is also desirable to have a interface to identify which parameters to save.