Better restrictions on manifolds

[FlorentinJ]

This ticket implements two new attributes of tensor fields on manifolds used to improve the restriction operation, by using rudimentary graph exploration to check for existing restriction and extensions.

Previously, defining the restriction of a tensor field on a subset V of a subset U before the restriction on the subset U would not register the first tensor as a restriction of the second, resulting in the creation of two python objects, despite being mathematically identical.

An example of code reproducing this behavior can be seen on this notebook. With this ticket, the order of creation no longer matters.

This is part of the ​SageManifolds project; see the metaticket #18528 for an overview.

CC: @egourgoulhon

Component: geometry

Keywords: restriction, extension, subdomain

Author: Florentin Jaffredo

Branch/Commit: cbbbf47

Reviewer: Eric Gourgoulhon

Issue created by migration from https://trac.sagemath.org/ticket/25417

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. New commits:

9700c8b

Merge branch with 8.3.beta2

[sagetrac-git]

Changed commit from 5bd6352 to 9700c8b

[egourgoulhon]

Reviewer: Eric Gourgoulhon

[egourgoulhon]

comment:3

LGTM.
Simply, could you add some Python comments in lines 426-427 of tensorfield.py regarding the meaning of the new attributes _extensions_graph and _restrictions_graph? This would be helpfull, especially since, technically speaking, these attributes are not graphs, but dictionaries.

[sagetrac-git]

Changed commit from 9700c8b to db9cd16

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. New commits:

db9cd16

Added some comments to explain the purpose of new attributes

[egourgoulhon]

comment:5

Thanks for the changes. LGTM. I think the failures in the plugins startup_time and startup_modules reported by the patchbot are irrelevant as far as this ticket is concerned.