Support Shape Inheritance

[calummackervoy]

Lots of shapes we'll be using are extensions of other shapes. For example, a Vampire Shape is normally an extension of a generic Character Shape

We should support an "include" or "extends" mechanism to allow the Generic Character to define the core stuff, and Vampire just to be concerned with those extensions which are important to Vampires

A priori the best way to do this in SHACL is to use rdfs:subClassOf on the sh:NodeShape. Then existing properties can be overridden, as well as new properties extended

I think that the SHACL libraries we're using don't do the inference automatically normally and we need to do it ourselves