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 
What is the advantage of this base class? It replaces:

```
def __init__(self, reply, informs):
    self.reply, self.informs = reply, informs
```

This also happens to be two lines. And you can remove the corresponding attributes.

I find base classes to be an unnecessary mental drag and they really have to shine to prove their worth.