``nickname`` attribute removed from ABCPolyBase
-----------------------------------------------

An abstract property ``nickname`` has been removed from  ``ABCPolyBase`` as it
was no longer used in the derived convenience classes.
This may affect users who have derived classes from ``ABCPolyBase`` and 
overridden the methods for representation and display, e.g. ``__str__``, 
``__repr__``, ``_repr_latex``, etc.