ENH add ThreeOperatorSplitting Solver

[mathurinm]

Article by Damek Davis and Wotao Yin to solve min f + g + h, all 3 convex (generalizes Froward Backward and Douglas Rachford)

also apparently work for non convex f: http://proceedings.mlr.press/v139/yurtsever21a/yurtsever21a.pdf