Update Norm.cu to fix z coordinate in NormBackgroundMonoKernel mask

[CACNYC]

Use the z loop variable when computing the centered z coordinate for the particle-radius background mask.

Fixes #481 Question about z-coordinate in NormBackgroundMonoKernel background mask

This PR applies a minimal one-line fix in:

NativeAcceleration/gtom/src/ImageManipulation/Norm.cu

Current code in NormBackgroundMonoKernel computes the centered z coordinate as:

int zz = y - dims.z / 2;

Surrounding loop iterates over z, y, and x, and the voxel access uses the actual z index:

tfloat val = d_input[(z * dims.y + y) * dims.x + x];

Proposed radius calculation uses the centered coordinates for all three dimensions:

int zz = z - dims.z / 2;
int yy = y - dims.y / 2;
int xx = x - dims.x / 2;

PR change:

- int zz = y - dims.z / 2;
+ int zz = z - dims.z / 2;