Add stub methods to TableColumn

The `TableColumn` class lacks a few methods that are assumed to exist as part of the API by the `TableEditor` (with `get_value` probably being the most important one). It would be good to stub these out, either with null implementations or raising `NotImplementedError` if they are required. Even better might be to make `TableColumn` a proper `ABCHasTraits` subclass with certain critical methods marked as `@abstractmethods`.