Restructure manifold module

[mjungmath]

I propose to restructure the manifold module. For example:

• separate the code related to the (pseudo-)Riemannian setting and dedicate a own module to it
• create a new module for vector bundles

Some things are currently very vague:

• operators.py clearly contains code related to differentiable manifolds but stays in the main module
• we have a submodule subsets but the separation is currently not very consistent; subset.py for example still belongs to the main module

CC: @egourgoulhon @tscrim @mkoeppe @tobiasdiez @nthiery

Component: manifolds

Issue created by migration from https://trac.sagemath.org/ticket/32274

[egourgoulhon]

comment:1

OK for this, provided there is some deprecation period (last time there were such changes, it broke codes relying on Sage sources, like the kerrgeodesic_gw package).

Regarding operators.py, I agree with your remark; however, I would keep it in the main module for convenience of the end user, from sage.manifolds.operators import curl being shorter than from sage.manifolds.differentiable.operators import curl.