Sketch KATResource API (and related classes) using ABCs

Extract and define the non-KAT specific parts of the katcorelib KATCP client API into this package. The generic KATCP implementation will also be moved here.

Plan is that the KAT-specific API will be elaborated in https://github.com/ska-sa/kattelmod to facilitate colaboration with SP and easing the implementation of dry-running. The implementation of KAT-specific clients will build on this base in katcorelib.

@ludwigschwardt 
OK, that seems useful enough... The point of KATReply is to add value to the basic (reply, informs), but this way someone can still handle it in its original form.