* Pivoting abstractions should not mix grouped
and non-grouped abstractions.

* Make Abstractor::Abstractable::pivot_abstractions
only include non-grouped abstractions.

* Abstractor::Abstractable::pivot_abstractions
should return abstractable entities that have
not been abstracted.

* Allow Abstractor::Abstractable::abstractor_subjects
to be filtered to grouped and non-grouped
abstractor subjects.


* Allow Abstractor::Abstractable::abstractor_abstraction_schemas
to be filtered to grouped and non-grouped
abstractor abstraction schemas.
Bump to version 1.0.13