support for inheritance in specifications

[primalmotion]

It would be nice instead of assuming that creationDate, owner etc are default bambou attributes, we could create a specification that is inherited.

I suggest that we could add some specifications prefixed by a symbol
@vsdobject.spec

The repo parsers would ignore those prefixed specifications when building the object model.

Then in standard specifications we could add in the model section an attribute extends: vsdobject

When a parser gets a spec, it would look for this token, then fetch the inherited specification, in order to correctly compute the Specification object.

It should also not difficult to allow multiple inheritence.

subnet.spec extends @vsdobject.spec extends @restobject.spec