Update documentation to reflect xija_gui_fit overhaul post #69

[taldcroft]

As a follow-on to #69, there are a bunch of changes to the GUI for fitting models that would benefit from some documentation:

• Added an IPython terminal which has access to several commands related to fitting, freezing and thawing, and setting of parameters. Examples
• Added "ignore" and "notice" options to the command box and the IPython terminal which allows one to mask out certain times from the fit, and remove those masks later. Allowed syntax
• Added functionality for adding limits to the JSON file. Where in the JSON to do this, allowed values, etc
• Added units to plots. Where in the JSON to do this, allowed values, etc

This belongs in:
https://cxc.cfa.harvard.edu/mta/ASPECT/tool_doc/xija/tutorial.html#fitting-a-model

And of course the picture of the GUI is out of date now, though getting that right is much less important than the basic usability updates above.

[jzuhone]

@taldcroft I started doing this in the past week.