Enable saving samples to hard drive for Approximation.sample, sample_prior_predictive, and sample_posterior_predictive

[twolodzko]

At this moment Approximation.sample, sample_posterior_predictive, and sample_posterior_predictive methods do not have trace parameters (save to trace) as pm.sample. They are hardcoded to gather the results as a large array in RAM. When sampling from large datasets (many parameters, many datapoints) the returning arrays grow in RAM and at some point become unusable, eventually leading to MemoryErrors. This could be solved by enabling user to choose way of storing the results on hard drive rather then memory, using the MultiTrace functionality.

[fonnesbeck]

First part finished with merge of #3557

[kc611]

I think in current master sample_posterior_predictive and similar methods do have trace parameters. So I guess this feature request is already statisfied.

[ricardoV94]

Closing in favor of #5371