Combinatorial designs: add some difference matrices and related objects

[gh-Ivo-Maffei]

We add a few constructions of difference matrices whose lambda parameter is not 1.

We then modify the orthogonal arrays and transversal designs constructions to take advantage of these additions. Finally, we add a new function symmetric_net.

[dimpase]

I see failing doctests:

sage -t --warn-long 94.4 src/sage/combinat/designs/latin_squares.py  # 2 doctests failed
sage -t --warn-long 94.4 src/sage/combinat/designs/designs_pyx.pyx  # 3 doctests failed

[git]

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

​6c362d8

fixed old doctests and bugs

[dimpase]

Do you run tests using GitHub Actions? (see #29401 for details)

In your code, to build docs, one needs

src/sage/combinat/designs/orthogonal_arrays.py

@@ -80 +80 @@
       Set to ``True`` by default
 
     - ``existence`` -- bolean. Instead of returnig a symmetric net, return:
-      - ``True`` -- such a net can be constructed by Sage
-      - ``False`` -- no such a net exists
-      - ``Unknown`` -- Sage does not know how to build such a design
-        so such design may or may not exist
+        - ``True`` -- such a net can be constructed by Sage
+        - ``False`` -- no such a net exists
+        - ``Unknown`` -- Sage does not know how to build such a design
+          so such design may or may not exist
 
     EXAMPLES::
 

src/sage/combinat/designs/difference_matrices.py

@@ -428 +428 @@
     - ``g,k,\lambda`` -- (integers) parameters of the difference matrix to construct
 
     - ``existence`` -- (boolean) instead of building the design, return:
-      - ``True`` if Sage can build the difference matrix using the subgroup construction
-      - ``False`` if Sage can't build the difference matrix using this construction
-         Note that Sage may be able to build such difference matrix in other ways
+        - ``True`` if Sage can build the difference matrix using the subgroup construction
+        - ``False`` if Sage can't build the difference matrix using this construction
+          Note that Sage may be able to build such difference matrix in other ways
 
     EXAMPLES::
 

(sphinx is very sensitive to correct indentation)

[mkoeppe]

Batch modifying tickets that will likely not be ready for 9.1, based on a review of the ticket title, branch/review status, and last modification date.

[slelievre]

One occurrence of "more than 1 time" should be "more than once" as elsewhere.

Can you do a round of pep8 formatting?

I would use lamda as an alternate name for lambda (since lambda is a reserved keyword in Python). I find it makes code easier to read than lambd or lmbda.