Implementation of the Demazure product in Coxeter groups

[stumpc5]

The provided patch implements the Demazure product of a word in the generators of a Coxeter group.

The definition can be found e.g. in "Knutson, Miller - Subword complexes for Coxeter groups".

            sage: W = CoxeterGroup(['A',2],index_set=[1,2])

            sage: w = W.demazure_product([2,2,1])
            sage: w.reduced_word()
            word: 21

            sage: w = W.demazure_product([2,1,2,1,2])
            sage: w.reduced_word()
            word: 121

            sage: W = CoxeterGroup(['B',2],index_set=[1,2])

            sage: w = W.demazure_product([2,1,2,1,2])
            sage: w.reduced_word()
            word: 2121

As I also needed it, the patch provides in addition a method returning the cartan type of a Coxeter group.

Depends on Ticket #8359.