Improvements to root systems

[tscrim]

Implement various things for affine root systems:

• The symmetric form ( | ).
• Methods for checking the short/long/imaginary roots.
• Modelling the positive roots roots in affine types.
• Basic untwisted types (so type A_2n⁽²⁾ would give A_2n).
• (Dual) Coxeter numbers for finite types.

CC: @sagetrac-sage-combinat @anneschilling @nthiery @darijgr @bsalisbury1 @dwbump

Component: combinatorics

Keywords: root systems, days54

Author: Travis Scrimshaw

Branch/Commit: 7285940

Reviewer: Daniel Bump

Issue created by migration from https://trac.sagemath.org/ticket/15384

[tscrim]

New commits:

[0fa598a](https://github.com/sagemath/sagetrac-mirror/commit/0fa598a)	imported patch root_system_data-ts.patch
[870837f](https://github.com/sagemath/sagetrac-mirror/commit/870837f)	imported patch root_systems-ts.patch

[tscrim]

Commit: 0fa598a

[tscrim]

Branch: public/combinat/root_systems/improvements-15384

[tscrim]

Changed keywords from root systems to root systems, days54

[sagetrac-git]

Changed commit from 0fa598a to e40f892

[sagetrac-git]

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

[e40f892](https://github.com/sagemath/sagetrac-mirror/commit/e40f892)	Changed horizontal to basic_untwisted.
[930782b](https://github.com/sagemath/sagetrac-mirror/commit/930782b)	Fixed indentation error.
[f87789d](https://github.com/sagemath/sagetrac-mirror/commit/f87789d)	Merge branch 'master' into public/combinat/root_systems/improvements-15384

[sagetrac-git]

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

[b4c7865](https://github.com/sagemath/sagetrac-mirror/commit/b4c7865)

Merge branch 'master' into public/combinat/root_systems/improvements-15384

[sagetrac-git]

Changed commit from e40f892 to b4c7865