HashTableNT: support adding and removing elements from the namedtuple? #31

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.