Satellite data support #15
Description
Textbook says that elements of a dynamic set may have satellite data as one on their fields. We already have Element class that consists of data field, but apart from 2 or 3 cases, the field is unused in the implementations and is not tested anywhere.
Decide if we want to support it, if so cover them with tests and make Element a superclass for any nodes class, which is part of the implementation of a dynamic set.