AutoPlot generation for model documentation does not include background

[butler]

Somewhere along the line in transitioning from backgrounds explicitly being defined in models and being managed by the sasmodels infrastructure the auto generation of the plot for documentation has lost that parameter. The result is that now the plot does not match the default parameters in the table of the same docs.

[richardh]

This is of course only seen if a new plot has to be generated following changes to a model, hence we found it whilst sorting #1112 for the Guinier model.

[pkienzle]

Fixed in sasmodels. Close ticket when verified.

[butler]

Fixed by commit 7b9e4ddca7ad3551d8fe3696d2afe75b0018599e. Tested that it does as advertized. Was pushed directly to master rather than through a PR as was a rather simple fix and sasmodels cannot yet have builds for testing. Basically created a DEFAULT_BACKGROUND and set to 1e-3 in direct_model.py rather than set it to 1e-3 in the two places it currently is set: direct_model.py and modelinfo.py)

Now closing ticket