clean function get_stacks_fourier()

[mathrip]

kspace_plane_loc and z_sample_loc represent the same thing : One is z_locations and other just indexes to these locations. Need to clean this function

kspace_plane_loc, z_sample_loc, sort_pos, idx_mask_z = \
            get_stacks_fourier(samples, img_shape)

[chaithyagr]

As idx_mask_z is something added as a part of #58 . I would prefer to fix this in this PR itself.

[mathrip]

We can close the issue as another one is already open for this function to change