FIX: Check Validity of QAbstractTableModel Classes

[zdomke]

This started with adding basic tests to check the validity of 3 QAbstractTableModel classes:

• BasePlotAxesModel
• BasePlotCurvesModel
• PyDMArchiverTimePlotCurvesModel

I fixed the issues that came up when the models were declared invalid. These fixes include:

• In flags(), the models check the validity of the given QModelIndex
• In data() and setData(), returns None instead of an empty QVariant object. This is done because QVariant is not really needed in PyQt like it is in Qt. In some cases where it makes more sense, I replaced the QVariant with empty strings.
  • While this documentation suggests returning an invalid QVariant, it doesn't specify what an invalid QVariant should be
  • This documentation suggests that None is an invalid QVariant, and that is what we should use
  • pytest-qt's qtmodeltester fixture that I used to test the models' validity checks that None is being returned for invalid data() requests, not QVariant

Also added basic docstrings to flags() methods.

[jbellister-slac]

Thanks!