This introduces typing for BaseConditionChoice and all subclasses.

All class attributes are clearly typed, as are instance variables.
This is a pretty straight-forward change. However, there are some issues

with the current design that prevents us from correctly typing the

default_value_field. In some cases, we set this as an attribute, and

in others as a function. That will need to be cleaned up as a separate

change.

Unit tests pass.