gh-37802: Add the downward monotonic cone to the cone catalogue
This an an old PR that I never migrated from trac. We have a catalogue
of convex cones accessible through `cones.foo()`, and this PR adds the
downward monotonic cone to the catalogue. It will also close
https://github.com/sagemath/sage/issues/30200.
URL: https://github.com/sagemath/sage/pull/37802
Reported by: Michael Orlitzky
Reviewer(s): Matthias Köppe
