Generalize automatic sigma calculations so that they can be used in both 3D and 4D cases

Created by: dyang37

As per discussion last week, the first step of implementing/validating mace4D is to modify auto_sigma_y so that the calculation can be generalized to 4D case.

This will not change the way sigma is calculated in 3D cases.

The real code change is just one line inside auto_sigma_y:

-    (num_views, num_det_rows, num_det_channels) = sino.shape
+    num_det_channels = sino.shape[-1]

The rest of the modifications are just docstring updates regarding the shape of sino in auto_sigma functions:

Tested on 3D shepp logan and MACE demo scripts. Results are visually the same.