Allow storing subset of variable in `pm.sample` via a `var_names` kwarg

[ricardoV94]

Description

Sometimes models with large Deterministics run out of RAM during sampling. One could compute the Deterministics afterwards easily, but would need to create the same model twice, once with the Deterministics and once without.

[ricardoV94]

Also a helper pm.compute_deterministics to compute any deterministics on demand