Add competing models and modify predictive fit

[marcdotson]

@RogerOverNOut this is yours, working on the predictive-fit branch.

• [x] Get predictive_fit_ensemble() working without reference to the global environment. Let @marcdotson know and I'll merge those changes into the simulation-tuning and ensemble-tuning branches.
• [x] Draw betas using upper-level covariates instead of averaging betas for predictive fit. Maybe as its own function?

Initial results, for reference:

Model	LOO	Hit Rate	Hit Prob
HMNL	-2732	0.566	0.446
Ensemble	-13.4	0.567	0.402

[marcdotson]

Merged changes into simulating-tuning and ensemble-tuning branches.

[marcdotson]

Merged changes into simulation-tuning.

[marcdotson]

Again, merged changes into simulation-tuning and ensemble-tuning.

[marcdotson]

Changed submitted with PR #57.