Fix bug in printing gaussian priors.  __str__ for mbb_fit_results
outputs information about lambda_peak prior and limits if present.
Add support for only reading in filter responses when needed.

* response_set now has an option to only read in/set up responses
  when actually needed.  This is off by default, but controlled
  by an argument to its initializer.  It is off by default because
  turning it on means that a pickled result moved to a different
  machine can't access filters not actually used in the fit.