Automatically detect globals in gen quants

[martinmodrak]

To use generated_quantities with parallel processing, the user currently has to manually provide a list of "globals": A list of names of objects that are defined in the global environment and need to present for the gen. quants. to evaluate. It is added to the globals argument to [future::future()], to make those objects available on all workers.

We should be able to detect those globals automatically by inspecting the syntax trees of the expressions (this is presumably how future does it's autodetction.

[martinmodrak]

It appears the globals package could make this a breeze (https://cran.r-project.org/web/packages/globals/index.html)