Minor improvements in error messages, return values

* Improve some of the error messages
* mbb_fit_results.data also returns the flux uncertainties.
* f500 added as a text synonym for fnorm
Make python3 compatible

* Make the same code base use python2.7 or python3+.  Does not use
   2to3.
* Bump version number to 0.3.0
* Some other minor fixes
Add routine to mbb_fit_results to randomly choose sample.

* Add mbb_fit_results.choice