access to iteration number and chain id within model

Sean O'Riordain suggested on stan-users that it would be useful to be able to access the iteration number in a Stan program so that you could write something like this to print theta every 100th iteration:

if (mod(iteration.count,100) == 0) print(theta);

It would also be nice to get the chain_id in case things are running in parallel.

This brings up the issue of what the values should be when, for instance, we're running diagnostics, just evaluating the log probability function, running optimization, etc. Maybe just -1 values everywhere?

There is also the issue of which blocks this should work in. Should prints in the transformed data block get a value of iteration=0 as the value? The chain_id would still work.

Begin by resolving the intended values for iteration and chain_id during diagnostics, log-probability evaluation, optimization, and each model block. Define the expected behavior for transformed data and printing, then use those decisions as the completion criteria for the feature.