Some code improvements in mixed_form.py and mixed_form_algebra.py.

This ticket is part of the metaticket #28519.

Depends on #28579

CC: @tscrim @egourgoulhon

Component: geometry

Keywords: mixed forms

Author: Michael Jung

Branch/Commit: 449e1b4

Reviewer: Eric Gourgoulhon

Issue created by migration from https://trac.sagemath.org/ticket/28578