Add a model viewer option to specify the model by variable name in __main__

[eslickj]

Summary/Motivation:

When you are working with the model viewer in Jupyter (Lab, qtconsole...) usually your Pyomo model is stored in main. This isn't necessarily the case, but it is more than likely, and if we assume that is the case we can do a few things to make the viewer easier to work with. This adds an option to specify the model by its variable name in main rather than directly specifying the model by pointer. This is just a new option and doesn't change the default behavior. In jupyter there is a callback that causes the model view to refresh every time you run a command. When you use the new option, the refresh will also check the model id to see if it has changed and load the new model if it has.

This also changes what happens if you open the UI without specifying a model. It looks in the main namespace and pulls out the first pyomo model it finds based on the order of dir(main), which I think is alphabetical. In the qtconsole-based model-viewer, it no longer creates an empty model if it can't find one, it just shows the viewer empty, and you'll have to point it at a model after you create one.

The main motivation for this is that it saves me (and hopefully anyone else that uses the viewer) a lot of time selecting the model in the UI. A lot of times I create a model by running a script. This lets me run the script and open the UI without explicitly selecting the model. Then I often find something need changed in the model which is easier done by updating the script. This lets me rerun the script overwriting the model and having the UI update without having to go back and select the model again. This also avoids confusion where the user may not realize they are looking at an older version of the model.

Changes proposed in this PR:

• Add option to automatically update the model in the viewer if it is replaced by a new model.
• Search the main namespace for a model, if one is not specified
• Don't create a new model when opening the UI in the qtconsole app if one is not specified.

Legal Acknowledgement

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1. I agree my contributions are submitted under the BSD license.
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[codecov]

Codecov Report

Attention: Patch coverage is 41.66667% with 21 lines in your changes missing coverage. Please review.

Project coverage is 88.55%. Comparing base (6fb0c5d) to head (19df8be).
Report is 1033 commits behind head on main.

Files with missing lines	Patch %	Lines
pyomo/contrib/viewer/ui.py	42.30%	15 Missing ⚠️
pyomo/contrib/viewer/model_select.py	25.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3271      +/-   ##
==========================================
+ Coverage   88.51%   88.55%   +0.03%     
==========================================
  Files         865      865              
  Lines       97585    97610      +25     
==========================================
+ Hits        86381    86440      +59     
+ Misses      11204    11170      -34     

Flag	Coverage Δ
linux	86.48% <41.66%> (-0.02%)	⬇️
osx	?
other	86.68% <41.66%> (-0.01%)	⬇️
win	83.96% <41.66%> (-0.01%)	⬇️

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