Multithreaded multiplication for algebras with basis

[elixyre]

I purpose a simple patch to improve the multiplication method in Algebras_with_basis category

[nthiery]

Salut!

Great idea. And very natural once you think about it.

I don't think this should be activated by default though. For simple product rules, or when there is a cache on the product on basis, this most likely induces a regression. On the other hand, this suggests that it should be possible to easily configure at least to options for products in an algebra with basis:

• Whether to cache the product on the basis
• Whether to use parallelism

There just remains to find and implement a good idiom :-)

On a technical note: it might be better to move the implementation of the @parallel-decorated method outside of the body of the methods to avoid recreating them each time. I am not sure as well of the logic to handle _multiply_on_basis (but that could be an occasion to finally deprecate it).

Cheers,

Nicolas