Fix coercions for descent and symmetric group algebras

[tscrim]

This splits off the first issue of #15475 (which has already been reviewed by Darij). This will also address the issues noted on https://groups.google.com/forum/#!topic/sage-devel/rXobHQXrCck.

CC: @darijgr

Component: coercion

Keywords: symmetric group algebra, descent algebra

Author: Travis Scrimshaw

Branch: d8ea764

Reviewer: Darij Grinberg

Issue created by migration from https://trac.sagemath.org/ticket/16678

[tscrim]

Branch: public/coercion/fix_sga_coercions-16678

[tscrim]

Description changed:

--- 
+++ 
@@ -1 +1 @@
-This splits off the first issue of #15475 (which has already been reviewed by Darij). This will also address some of the issues noted on https://groups.google.com/forum/#!topic/sage-devel/rXobHQXrCck.
+This splits off the first issue of #15475 (which has already been reviewed by Darij). This will also address the issues noted on https://groups.google.com/forum/#!topic/sage-devel/rXobHQXrCck.

[tscrim]

Commit: e1ff769

[tscrim]

Last 10 new commits:

7637ab2	Implemented `_coerce_map_from_` for SGA.
0763ba5	More fixes and doctests.
db24a90	Fixed the issue...finally... Also some other minor cleanup.
fb8c9c9	Added comment about register_embedding.
702e7a1	Changed to use `_coerce_map_from_` and made it more robust/general.
f662ae3	documentation fixes to canonical_embedding of symmetric group algebra (yes, these maps aren't always embeddings, and the inequality was backwards)
ff4e546	oops, another doc fix
b3d98ba	Merge commit 'ff4e546a838e9e9a4be86b3a89d8596658beb3f4' into public/coercions/fix_sga_coercions-16678
31c8ac9	Additional fixes noted on sage-devel thread.
e1ff769	Fixed stupid mistake.

[sagetrac-git]

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

30e769e

Removed erroneous character in permgroup.py.

[sagetrac-git]

Changed commit from e1ff769 to 30e769e

[sagetrac-git]

Changed commit from 30e769e to 3fa802d

[sagetrac-git]

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

3fa802d

a couple doctests and a blunder of my own

[darijgr]

comment:4

Thanks for the split, which was rather appropriate. Positive_review on my side.