ManifoldSubset: Replace use of _top_subsets by a generator

[mkoeppe]

... of the maximal elements of the subset poset.

Depends on #31736
Depends on #31764

CC: @mjungmath @egourgoulhon

Component: manifolds

Issue created by migration from https://trac.sagemath.org/ticket/31766

[mkoeppe]

Changed dependencies from #31736 to #31736, #31764

[mkoeppe]

comment:2

Actually _top_subsets is only used in one place, in TensorField.along.

Perhaps it is possible to get rid of it altogether?

[mjungmath]

comment:3

Oh, something similar happens for the volume form and the display method of scalar fields. Moreover, see _get_min_covering in manifold.py.