so that Jupyter notebook can show them nicely

https://ipython.readthedocs.io/en/stable/api/generated/IPython.core.formatters.html#module-IPython.core.formatters

To test, open the notebook and try

PP = PlanePartition([[4,3,3,1],[2,1,1],[1,1]]);PP

CC: @tscrim @slel

Component: combinatorics

Author: Frédéric Chapoton

Branch/Commit: 96539b4

Reviewer: Travis Scrimshaw

Issue created by migration from https://trac.sagemath.org/ticket/34646