Add a function to extract single draw of multidimensional variable about posterior

[mjskay]

Thinking about how this relates to uses of variable names across the API, this seems related to the inconsistent handling of indices in variable names between draws_rvars and the other formats (see here: #208 (comment))

One thought is to fix the above-referenced issue so that something like variables(x, with_indices = TRUE) (default) returns names like x[1,1], x[1,2], ... and variables(x, with_indices = FALSE) returns names like x. Then have clear indications in the API of which functions take variable names with or without indices.

For this issue specifically, following the pattern of extract_variable_matrix() we could have an extract_variable_array(x, variable) that takes a variable name without indices and returns its array form. Then the solution in the above example would be extract_variable_array(x, "z_1")[1,,].

If we also want a solution specifically for getting one draw regardless of shape, then there is helper code used by for_each_draw that could be extracted and adapted for that purpose.

[mjskay]

Cool, happy to take a stab at it.

[avehtari]

I did also notice the index-inconsistency, and like the idea of with_indices

extract_variable_array would work for me (the first use case is stan-dev/cmdstanr#876)

[paul-buerkner]

I agree. Sounds like a good solution.