Clean up and document Keys

[harvey2phase]

Ideally, each particular key in Key class refers to the same variable no matter the context and dictionary: if two dictionaries contain the same object/data, they can share the same key; the same key should not reference different data types/structures/values.

Once the above is done, then each key can be documented in either the Key class or a new Wiki page (or short description in the former and longer one in the latter).