Conflicting definition of proxmal parameters `rho` and `theta`.

[antonysigma]

In the folder proximal/prox_fns and proximal/algorithms, the proximal functions are defined as:

$$\mathrm{prox}_f (v) = \arg \min_x f(x) + \frac{\rho}{2} \Vert x - v \Vert_2^2,$$

whereas in the folder proximal/halide/, the functions are defined as:

$$\mathrm{prox}_f (v) = \arg \min_x f(x) + \frac{1}{2\theta} \Vert x - v \Vert_2^2,$$

which causes the Halide-generated, end-to-end L-ADMM solver to fail.

Action: Unify all definitions to avoid mistakes, to simplify code, and to avoid having to convert between theta and rho frequently.

Also theta symbol is already taken by the Pock-Chambolle algorithm. Use tau instead.