Add a method number_of_terms for all laurent polynomials. To do so, add such a method for multi polynomials as well (number_of_terms for univariate polynomials exists already).

Component: algebra

Author: Daniel Krenn

Branch/Commit: 4dc419d

Reviewer: Frédéric Chapoton

Issue created by migration from https://trac.sagemath.org/ticket/21966