trac #14095 remove use of .unit ; added cartan_type for SymmetricGroup
One more unit() to one().
Some misc cleanup to permgroup_named.py.
changed the forward slash to backslash for MatLab operator
trac #14095 added the new example from the trac ticket
correct formatting of doc and raise syntax for python3
more doc correction
Set() with no argument returns an empty Set.
trac #14095 cdots instead of dots
Trac #14095: SymmetricGroup as a proper WeylGroup

During Sara Billey's talk, it came up that not all of the Weyl group
functionalities are available for the symmetric group:

- The symmetric group does not implement CartanType
- The Stanley symmetric function code uses the old fashionned W.unit()
rather than W.one(). There might be some other occurences that should be
cleaned up

Test case:
{{{
sage: G = SymmetricGroup(4)
sage: w = G.from_reduced_word([3,2,3,1])
sage: w.stanley_symmetric_function()
3*m[1, 1, 1, 1] + 2*m[2, 1, 1] + m[2, 2] + m[3, 1]
}}}

URL: http://trac.sagemath.org/14095
Reported by: nthiery
Ticket author(s): Frédéric Chapoton
Reviewer(s): Travis Scrimshaw
Trac #16090: Set() should return an empty Set

`Set` with no argument is currently an error. It should be changed to
follow what Python's built-in container constructors do: return an empty
container.

URL: http://trac.sagemath.org/16090
Reported by: mathzeta2
Ticket author(s): Tomer Bauer
Reviewer(s): Nathann Cohen
Trac #15939: Typo on backslash operator documentation: uses regular slash instead of backslash

sage/misc/preparser.py, line 1346:

Says:

{{{
    Implements Matlab-style backslash operator for solving systems::

        A / b
}}}
Should say:

{{{
    Implements Matlab-style backslash operator for solving systems::

        A \ b
}}}

URL: http://trac.sagemath.org/15939
Reported by: cousteau
Ticket author(s): Volker Braun
Reviewer(s): Matthieu Dien