Fix instabilities in xija_gui_fit, add ability to adjust error limits in histogram plots

[jzuhone]

Description

xija_gui_fit occasionally exhibits random crashes, especially when one is attempting to make new plots or pan/zoom within existing plots. This appears to be due to race conditions where an instruction to change a plot is made to a plot object that no longer exists.

Along with https://github.com/sot/Ska.Matplotlib/pull/25, this PR seeks to avoid these crashes by doing the following:

• Avoid creating and re-creating Figure objects as much as possible, instead save the Figure object and update its data
• Add a short delay before updating plots after adding limit lines
• Caching rad zone times so they don't need to be created the first time you plot them
• General cleanup and simplification of code

This PR also adds the ability to manually specify the error limits in histogram plots, see the new input boxes in the lower-right of this image:

This allows more control over the histogram plots to make them more reproducible.

Testing

Unit tests

• [x] No unit tests
• [ ] Mac
• [ ] Linux
• [ ] Windows

Functional tests

• Open up xija_gui_fit, fit a model, create plots, zoom in and out, pan, re-fit, etc. to see if one gets a crash--no crashes occurred
• Check that changing the error plot limits update plots as desired

[jzuhone]

any comments on this PR?

[taldcroft]

@jzuhone - feel free to merge.