Ticket 16835: added CartesianProducts and TensorProducts for the category of vector spaces with basis.
Ticket 16835: make VectorSpaces also closed under CartesianProducts and TensorProducts.
Ticket 16835: fix documentation
Ticket 16385: changed about a million doctests to reflect the changed categories graph.
Merge branch 'u/cnassau/16835' of trac.sagemath.org:sage into u/tscrim/product_modules_with_basis-16835
Added some line breaks.
Added a quick check for number of vertices and edges.
Trac #16835: cartesian product of VectorSpaces not in VectorSpaces

{{{
sage: X=CombinatorialFreeModule(RDF,ZZ,category=ModulesWithBasis(RDF))
sage: Y=cartesian_product((X,X))
sage: X in ModulesWithBasis(RDF)
True
sage: Y in ModulesWithBasis(RDF)
False
sage: Y.categories()[1]
Category of modules with basis over Real Double Field
sage: Y.categories()[1] is ModulesWithBasis(RDF)
False
}}}

This weirdness relates to the fact that `ModulesWithBasis(RDF)` prints
as the `Category of vector spaces with basis over Real Double Field`.
The `Category of modules with basis over Real Double Field` is one of
its super categories.

In the case at hand I cannot construct a morphism from `Y` to `X`
because of the category mismatch:
{{{
sage: Y.module_morphism(codomain=X,on_basis=lambda (i,j):X.monomial(i))
------------------------------------------------------------------------
---
ValueError                                Traceback (most recent call
last)
..
ValueError: Free module generated by Integer Ring over Real Double Field
(+) Free module generated by Integer Ring over Real Double Field is not
in Category of vector spaces with basis over Real Double Field
}}}

URL: http://trac.sagemath.org/16835
Reported by: cnassau
Ticket author(s): Christian Nassau
Reviewer(s): Travis Scrimshaw
Trac #16984: Poset is_chain: Minimal speedup

`is_chain()` for Poset can first check that number of vertices is
exactly one more than number of edges.

URL: http://trac.sagemath.org/16984
Reported by: jmantysalo
Ticket author(s): Jori Mäntysalo
Reviewer(s): Nathann Cohen