16340: infrastructure for modelling full subcategories
16340: added full_super_categories method, a test, and some doc
16340: trivial ReST fix
Merge branch 'develop' into categories/full-subcategories-16340

Easy conflict:
	src/sage/categories/magmas.py
#10668: working draft of refactored support for morphisms and homsets
10668: specify that NN is commutative
10668: CategoryWithAxiom: simplified _without_axioms logic and improved repr

- _without_axioms is better expressed recursively

- Added a heuristic to try not to print out those axioms that are
  implied by others already printed out.
10668: Removed unused import
10668: Implement EndSets; construct the abstract element class in the homset not the homset category

Also add sage/categories/homsets.py which was missing in the previous commits.
10668: Trivial comment addition in Sets.ElementMethods
10668: Fixed refinement of category to handle Parent._abstract_element_class
10668: Simplify the logic of endsets of modular abelian varieties using the new Endsets construction
10668: trivial doctest output updates w.r.t. the new printing of homsets
10668: Explicitly document that Element.__getattr__ can also be useful when refining categories
10668: Fixed doctest
10668: fixed ReST typo
10668: trivial ReST fix
16340: super_structure_categories -> all_structure_super_categories
Merge branch 'develop' into categories/full-subcategories-16340
Merge branch 'categories/full-subcategories-16340' and '6.3beta5' into categories/morphism-methods-10668