Although subclasses of ConvexSet_base are not necessarily Parents, it will be useful for them to provide an_element and some_elements methods.

Depends on #31959

CC: @kliem @tscrim @yuan-zhou

Component: geometry

Author: Matthias Koeppe

Branch/Commit: 1744ffa

Reviewer: Travis Scrimshaw

Issue created by migration from https://trac.sagemath.org/ticket/31990