HashTableNT: support adding and removing elements from the namedtuple?

Think about whether we want such functionality in `HashTableNT` **or** if such stuff rather belongs elsewhere (like in the `HashTableNT` users code):

- dropping an element and loading an old ht that still has it is relatively straightforward
- adding an element and loading an old ht that does not yet have it is more difficult: would need a default value for the new element.