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.


beta > 0

we always put spaces around operators.