Accelerated coordinate descent for elastic net

This contribution implements the accelerated coordinate descent method for the elastic net based on the paper 
Fercoq, O., & Richtárik, P. (2013). Accelerated, parallel and proximal coordinate descent. arXiv preprint arXiv:1312.5799.
It does not replace the existing coordinate descent code although I also propose some marginal improvements.


don't add a new example. Eventually update the benchmark script in benchmark folder.

you'll need to report graph and computation times to justify this new feature.