Implementation of the SubwordComplex as defined by Knutson and Miller

[stumpc5]

This patch provides an implementation of the subword complex:

Fix a Coxeter system (W,S). Let Q = be a finite word in S and pi in W.

The subword complex Delta(Q,pi) is then defined to be the simplicial complex with vertices being {0,...,n-1}, (n = len(Q), one vertex for each letter in Q) and with facets given by all (indices of) subwords Q' of Q for which Q\Q' is a reduced expression for pi.

    sage: W = CoxeterGroup(['A',2],index_set=[1,2])
    sage: w = W.from_reduced_word([1,2,1])
    sage: C = SubwordComplex([2,1,2,1,2],w); C
    Subword complex of type ['A', 2] for Q = [2, 1, 2, 1, 2] and pi = 121
    sage: C.facets()
    {(1, 2), (3, 4), (0, 4), (2, 3), (0, 1)}

Component: combinatorics

Keywords: subword complex, simplicial complex

Author: Christian Stump

Branch/Commit: 17518c1

Reviewer: Frédéric Chapoton

Issue created by migration from https://trac.sagemath.org/ticket/11010

[stumpc5]

Author: Christian Stump

[stumpc5]

Description changed:

--- 
+++ 
@@ -13,6 +13,4 @@
     {(1, 2), (3, 4), (0, 4), (2, 3), (0, 1)}
 ```
 
-depends on Ticket #8359.
-
-I will upload the patch as soon as the patch on Coxeter groups is ready.
+Depends on Tickets #8359 and #11122.

[stumpc5]

Changed keywords from none to subword complex, simplicial complex

[stumpc5]

Description changed:

--- 
+++ 
@@ -5,12 +5,10 @@
 The subword complex Delta(Q,pi) is then defined to be the simplicial complex with vertices being {0,...,n-1}, (n = len(Q), one vertex for each letter in Q) and with facets given by all (indices of) subwords Q' of Q for which Q\Q' is a reduced expression for pi.
 
 ```
-    sage: W = CoxeterGroup(['A',2])
+    sage: W = CoxeterGroup(['A',2],index_set=[1,2])
     sage: w = W.from_reduced_word([1,2,1])
     sage: C = SubwordComplex([2,1,2,1,2],w); C
-    Subword complex of type ['A', 2] for Q = [2, 1, 2, 1, 2] and pi = [1, 2, 1]
+    Subword complex of type ['A', 2] for Q = [2, 1, 2, 1, 2] and pi = 121
     sage: C.facets()
     {(1, 2), (3, 4), (0, 4), (2, 3), (0, 1)}
 ```
-
-Depends on Tickets #8359 and #11122.

[stumpc5]

Dependencies: 11187, 11122

[stumpc5]

Changed dependencies from 11187, 11122 to #11187, #11122

[stumpc5]

Changed dependencies from #11187, #11122 to #11122

[fchapoton]

Changed dependencies from #11122 to #12774