Creating table misses fields defined on ancestor classes

When fastlite creates a table from a subclass, it does not create columns for attributes defined only on the parent class. https://colab.research.google.com/drive/1DIo0DIjXQM6t1EfUiVDgALP715YpfDZa?usp=sharing illustrates this problem, identifies its cause, and provides a potential solution. Let me know what you think. I'd be happy to make a PR if desired!