Class "unit" and "viewport" as S4 classes

Package "hexbin" now uses S4 classes throughout and hence needs to setOldClass both "unit" and "viewport" (which are S3 classes from the grid package), in order to be able to use those in slots of its own classes.

Objects from the Class

A virtual Class: No objects may be created from it.

Extends

Class "oldClass", directly.

Methods

No methods defined with class "unit" in the signature.