Tweak comments and TODO
Refactoring: replace 'field_choices' in field description with 'field_choice_labels' and 'field_choice_links'
Eliminate 'options' paramneter from bound_field constructor, and 'field_options_valkey' from FieldDescription:
their functions are now provided by 'field_choice_labels' in FieldDescription.
Fixed bug with entity reference links in list displayt (issue #37)
Refactored handling of enumerated values in view fields:  earlier ad hoc mechanisms have been
removed, with everything handled through maps defined in the FieldDescription value.
As a consequence, the options parameter to the bound_field constructor has been removed.