Progress towards chi^2 fitting in Python and more

Various and sundry improvements over the past few days:

* Parameter files read/write updated to 2.19 (and back compatible to 2.15)
* Refactoring and improvements to models and observations classes
* Improved plotting
* Basic infrastructure for chi^2 calculations in SED mode
* Improved documentation. See http://www.stsci.edu/~mperrin/software/mcfostpy/