Faster proxs and omegas

[daducci]

This PR should be able to enable the following speed-up on the computation of omega and prox functions:

• about 450x for prox_group_sparsity (from about 20.5 s to 48.2 ms)
• about 1400x for omega_group_sparsity (from about 15.6 s to 11.2 ms) The acceleration is achieved through a redesign of the data structure about the groups which allows a faster direct access to memory via cython.

[daducci]

@MarioOcampo , can you please check if this modification enable similar accelerations on your computer?

[MarioOcampo]

@daducci, the acceleration in my computer is:

• prox_group_sparsity: about 80x (from about 16.6 s to 196.2 ms)
• omega_group_sparsity: about 400x (from about 12.7 s to 32.6 ms)