maximum relevant lambda for tv1_1d

[anntzer]

For the TV1_1D case, it is well-known that the solution becomes completely constant at a finite value of lambda, which is (IIRC) the maximum absolute value of (D.Dt)^-1(Dx) (where D is the difference matrix). Such a function is easy to implement oneself, but perhaps it could be added as a helper function to prox_tv?