Prevent pathfinder when 0 parameters, warn if too many PSIS requested

[WardBrian]

Submisison Checklist

• Run tests: ./runCmdStanTests.py src/test
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Summary:

Closes #1208: warning when more PSIS draws are requested than total number of draws from single-pathfinders
Closes #1220: disallows pathfinder when the model has no parameters.

Arguably these could be put in the stan-dev/stan repo. I'm open to either

Documentation:

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[mitzimorris]

Arguably these could be put in the stan-dev/stan repo. I'm open to either

Error checks should go closest to where the error is made - since these are bad configs/requests, CmdStan is fine.

Long term, putting all the config checking into Stan makes sense - in the meantime, more duct tape!

[mitzimorris]

lgtm!
thanks!