Rename ValueTuple to ReadingTuple for consistency

Address @ludwigschwardt comment in https://github.com/ska-sa/katcp-python/pull/37
Well, actually the problem is a bit more widespread... Simon and Maciej introduced a "value tuple" all through the code (e.g. self._value_tuple) but now you have called essentially the same thing a "reading". Consistency is my hobgoblin here...