addKey method

[JohnAD]

Add a 'addKey' method that corresponds to the 'dropKey' method. Be careful when the LOD is actually a list of class instances.

[JohnAD]

Mostly done. However, further work is probably warranted. The current version of 'addKey' cannot
handle key-lists. In fact, it pretty much only works on true dictionaries with one level.

[JohnAD]

Should support passing a function to build the keys value.