[gitignore] added demos-*
[products.base] added notes
[products.elliptic] minor reorder

* put traits and base classes in internal namespace
* clean up a bit
[products.elliptic] prepare for diffusion factor and tensor

Extended the signatures an implemented the current variant as a
specialization.
[products.elliptic] added first draft of a 2nd variant to playground

These variants take a diffusion factor and a diffusion tensor.