Relationship between an extension and it's base class is not asserted

[EricBoix]

The creation of an extension requires two things

1. to specify how a base class of the 3Dtiles standard gets extended (with which attributes...)
2. to specify which base class is allowed to be extended in this way.

For example the

1. a BatchTableHierarchy extension extends "some" base class with classes, instancesLength... attributes
2. BatchTableHierarchy is an extension of a BatchTable

The problem: currently, the implementation doesn't allow for the expression of the relationship between an extension and the base classes it decorates (as the permissivity of ThreedtilesNotion::add_extension() illustrates it).

The current kludge: have this relationship expressed by the semantics of the schema names.