Add general SDMM operator

[mdavezac]

General framework seeks to minimize $\sum_i g_i(L_i*x)$. It takes as input a list of operators consisting of:

• L_i
• L_i^\dagger
• the proximal of g_i

It might/can also take as input a conjugate gradient method, a convergence criteria, itermax, gamma, ... something else?

[mdavezac]

I will be testing with $g_i = ||L_i(x - x0)||_2$.

Note to self: for i=0,1 $L_i = I$, the minimum is any point on the segment $x_0$ to $x_1$. Not just $0.5x_0 + 0.5x_1$.