Moved lir, dustmass, peaklambda computation to mbb_fit_results.

Code runs, not extensive testing.
Add ability to predict flux at a given wavlength from chain.

* Add methods predflux_cen and _predict_flux to mbb_fit_results.
  These aren't stored as part of the structure, unlike lir, etc.
* Fix bug where _has_lir wasn't set.
Added ability to specify response function name to predflux_cen.