Bundle connections were implementet quite superficially (by me). In this ticket, I want to improve the assigment behavior of bundle connections.

First of all, the methods set_connection_form and add_connection_form now return the corresponding connection 1-form ready for further assignment.

The main improvement in this ticket is to allow list assignments:

sage: M = Manifold(2, 'M', start_index=1)
sage: X.<x,y> = M.chart()
sage: E = M.vector_bundle(2, 'E')
sage: nab = E.bundle_connection('nabla')
sage: e = E.local_frame('e')
sage: nab[e, :] = 0  # set all entries to zero
sage: nab[e, 0, 0].display()
connection (1,1) of bundle connection nabla w.r.t. Local frame (E|_M, (e_1,e_2)) = 0

For details see the documentation, which I have also improved.

Depends on #30191

CC: @egourgoulhon @tscrim

Component: manifolds

Author: Michael Jung

Branch/Commit: d3c8cab

Reviewer: Travis Scrimshaw

Issue created by migration from https://trac.sagemath.org/ticket/30208