Handle differing-length chain and logls in chain_to_common_posterior.py
Added PSD parameters to posterior.py; seems to work (but run.py broken for now).

As a consequence, had to re-factor params.py code into posterior.py
Removed t_steps from run.py, since that code is now in emcee's PTSampler.
Removed useless pool-clearing code in malmquist_snr adjustment.
Revert "Removed t_steps from run.py, since that code is now in emcee's PTSampler."

This reverts commit 7c28a5e37c82bca5793fb350c2e4275dadf10af9.
Updated chain_to_common_posterior.py to ignore PSD parameters.
run.py and posterior.py work, but likelihoods returned are crazy.
Included d*d term in likelihood (needed for noise model), and added NoiseOnlyPosterior.
Slightly better best-params output in run.py