Implement generic support for parents with (multiple) realizations

[jbandlow]

This patch implement generic support for parents with (multiple) realizations
See:

    Sets().WithRealizations().example()

Depends on #12484
Depends on #12464

CC: @sagetrac-sage-combinat

Component: categories

Keywords: Cernay2012

Author: Nicolas M. Thiéry

Reviewer: Simon King, Florent Hivert

Merged: sage-5.0.beta11

Issue created by migration from https://trac.sagemath.org/ticket/7980

[jbandlow]

Description changed:

--- 
+++ 
@@ -1 +1,3 @@
 This code currently exists on the sage-combinat queue, in the ncsf patch.  It should be put in a more natural place in the category code.
+
+I'm recording here the decision made at sage days 20.5 to use the names `Representation` and `ParentWithRepresentations`.

[mwhansen]

comment:2

Didn't we want Realization rather than Representation?

[nthiery]

Description changed:

--- 
+++ 
@@ -1,3 +1,3 @@
 This code currently exists on the sage-combinat queue, in the ncsf patch.  It should be put in a more natural place in the category code.
 
-I'm recording here the decision made at sage days 20.5 to use the names `Representation` and `ParentWithRepresentations`.
+I'm recording here the decision made at sage days 20.5 to use the names `Realization` and `ParentWithRealizations`.

[nthiery]

Author: Nicolas M. Thiéry

[nthiery]

Description changed:

--- 
+++ 
@@ -1,3 +1,6 @@
-This code currently exists on the sage-combinat queue, in the ncsf patch.  It should be put in a more natural place in the category code.
+See patch under development on the Sage-Combinat queue:
 
-I'm recording here the decision made at sage days 20.5 to use the names `Realization` and `ParentWithRealizations`.
+http://combinat.sagemath.org/patches/file/tip/trac_7980-multiple-realizations-nt.patch
+
+Status: needs finalization
+