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.


it does not seem that you test all the code you added.

You should test the 3 solvers with dense and sparse data and report tiny duality gaps.