palp is a standard package, but with modularization that does not mean that everyone has it. In particular, sagemath-polyhedra (#32432) will not ship it.

We add a feature test along the lines of the palp spkg-configure.m4 script and mark many doctests in sage.geometry as # optional - palp.

To help construct doctest examples without having to rely on palp, we endow several classes with _sage_input_ methods.

As a cosmetic change, nef-partitions now use a unicode symbol.

-            Nef-partition {0, 1, 3} U {2, 4, 5}
+            Nef-partition {0, 1, 3} ⊔ {2, 4, 5}

We also add feature tests for polytopes_db, polytopes_db_4d.

CC: @kliem @novoselt @seblabbe @dimpase

Component: refactoring

Author: Matthias Koeppe

Branch/Commit: d97479d

Reviewer: Dima Pasechnik

Issue created by migration from https://trac.sagemath.org/ticket/32893