Allow drawing from the Laplace approximation and Pathfinder without evaluating log_p about stan HOT 4 OPEN

I'm OK with providing this option. Some notes:

• do we drop log_p and log_q both or provide independent options to drop either?
• I would strongly prefer to drop columns in the output rather than including dummy values to prevent people from trying to use it
• Our HMC sampler always provides log_p (and never provides log_q), so code that always assumes it exists will go astra
• log_q takes about the same amount of time as drawing a sample given that we have Cholesky factored already (i.e., it's a high cost)

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• For simplicity I would drop both
• I suggested dummy values as I assumed there can be code assuming their existence, but I'm fine dropping the columns, too

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Also adding Pathfinder for this to not evaluate lp__ for the return samples for each pathfinder

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