Implement untwisted affine Lie algebras

[tscrim]

Part of #14901. Implement untwisted affine Lie algebras from their corresponding classical one (given by extending the corresponding current algebra).

Depends on #16821

CC: @bsalisbury1

Component: algebra

Keywords: lie algebras

Author: Travis Scrimshaw

Branch/Commit: 3cf54ac

Reviewer: Ben Salisbury

Issue created by migration from https://trac.sagemath.org/ticket/16825

[tscrim]

comment:1

Preliminary version.

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Last 10 new commits:

921e2ca	Adding back support for creating a Lie algebra from a Cartan type.
721632b	Fixing bugs in structure coefficients of Chevalley basis.
23c1474	Implementing matrix Lie algebras in types E6, F4, and G2.
3862a22	Merge branch 't/16820/public/lie_algebras/fd_structure_coeff-16820' into t/16821/public/lie_algebras/classical-16821
abcf1e5	Fix citation problem
600a87a	Merge branch 'public/lie_algebras/classical-16821' of git://trac.sagemath.org/sage into public/lie_algebras/classical-16821
5cbde34	Doing rebasing due to changes in base ticket.
e3e309e	Merge branch 'develop' into public/lie_algebras/classical-16821
a7cede7	New data structure for elements.
321a6b3	Adding affine method and fixing highest root element for matrix Lie algebras.

[tscrim]

Commit: 321a6b3

[tscrim]

Branch: public/lie_algebras/untwisted_affine-16825

[sagetrac-git]

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

c75ce5e

Finishing initial implementation of untwisted affine Lie algebras.

[sagetrac-git]

Changed commit from 321a6b3 to c75ce5e

[sagetrac-git]

Changed commit from c75ce5e to a160cfd

[sagetrac-git]

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

a160cfd

Adding some coercions and exposing some more data.