Implement algebraic shifting operation for simplicial complexes

[jplab]

This tickets implements the exterior and symmetric shifting of simplicial complexes.

For more details on this operation, see for example:

https://arxiv.org/abs/math/0303233 https://arxiv.org/abs/0709.3265

where the operation is discussed in detail.

CC: @jplab @mo271 @videlec @jhpalmieri @tscrim

Component: geometry

Keywords: simplicial complexes, shifting

Author: Jean-Philippe Labbé

Branch/Commit: u/jipilab/shifting @ 48f7294

Issue created by migration from https://trac.sagemath.org/ticket/25868

[7ed8c4ca-6d56-4ae9-953a-41e42b4ed313]

Commit: 547760d

[7ed8c4ca-6d56-4ae9-953a-41e42b4ed313]

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

11956b8	First version of k-shift
cfe2872	some more improvement on shift
99c3e4e	changed method for shifting
547760d	First draft of exterior shifting

[jplab]

comment:2

I am still working on the function, but thought that I could upload my current working version.

I still would like to implement the symmetric shifting as well.

[7ed8c4ca-6d56-4ae9-953a-41e42b4ed313]

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

4f390bc	Merge branch 'shifting' into shiftingv83
48f7294	Repaired counter and added references

[7ed8c4ca-6d56-4ae9-953a-41e42b4ed313]

Changed commit from 547760d to 48f7294