ENH add "split" discretization

[mathurinm]

Following:

• Chambolle, Antonin, and Thomas Pock. "A remark on accelerated block coordinate descent for computing the proximity operators of a sum of convex functions." The SMAI Journal of computational mathematics 1 (2015): 29-54.
• Chambolle, Antonin, Pauline Tan, and Samuel Vaiter. "Accelerated alternating descent methods for Dykstra-like problems." Journal of Mathematical Imaging and Vision 59 (2017): 481-497.

This is an alternative discretisation of the continuous 2D TV friendly to GPU computation.