The zero element is always a special element. Therefore it should be treated as such. It should shorten computations and certainly be immutable. This ticket is devoted to that topic. (Similarly for the one element in the scalar field and mixed form algebra).

This ticket is part of the metaticket #28519.

Features

• an assertion error arises when altering the fixed elements zero or one
• a new attribute _is_zero is added to tensor fields and mixed form (similar to scalar fields)
• computations with involved zero or one are shortened by using a simple check
• due to immutability of algebra elements, no copies are returned anymore for scalar field operations with zero or one
• _is_zero attribute is applied for copies

CC: @tscrim @egourgoulhon

Component: geometry

Keywords: tensor fields, scalar fields, mixed forms

Author: Michael Jung

Branch/Commit: f2928f8

Reviewer: Eric Gourgoulhon, Travis Scrimshaw

Issue created by migration from https://trac.sagemath.org/ticket/28562