Trac #18678: Implement convolution_product for HopfAlgebras

In the study of Hopf algebras (both graded and finite dimensional), one
is led to look at convolution powers of the identity, and convolution
products more generally. These should be implemented in `bialgebras.py`.

Of possible future utility is the iterated coproduct, so I define that
as well.

URL: http://trac.sagemath.org/18678
Reported by: alauve
Ticket author(s): Aaron Lauve, Jean-Baptiste Priez, Amy Pang, Travis
Scrimshaw
Reviewer(s): Franco Saliola, Travis Scrimshaw

Author: Release Manager

src/sage/categories/bialgebras.py

@@ -12,6 +12,7 @@
 from sage.categories.category_types import Category_over_base_ring
 from sage.categories.all import Algebras, Coalgebras
 from sage.categories.super_modules import SuperModulesCategory
+from sage.misc.lazy_import import LazyImport
 
 class Bialgebras(Category_over_base_ring):
     """
@@ -60,3 +61,5 @@ def additional_structure(self):
     class Super(SuperModulesCategory):
         pass
 
+    WithBasis = LazyImport('sage.categories.bialgebras_with_basis', 'BialgebrasWithBasis')
+