add support for anisotropic values of the flow3D_smooth

[yuriyzubov]

Currently flow3D_smooth only accepts a single integer, applying the same Gaussian smoothing sigma uniformly across all spatial axes (Z, Y, X).

If voxel spacing differs across axes, passing a vector value to the flow3D_smooth parameter (anisotropic smoothing) makes more sense.

Changes

• flow3D_smooth now accepts int or list[int] (length 3)
• If an int is passed, it is broadcast to [val, val, val] for backwards compatibility
• Updated docstring to reflect the new accepted types

Example

# Uniform smoothing
model.eval(img, do_3D=True, flow3D_smooth=2)

# Anisotropic smoothing
model.eval(img, do_3D=True, flow3D_smooth=[3, 0, 0])  

[mrariden]

@yuriyzubov I think it makes more sense to scale the flow3D_smooth's first term by anisotropy inside of CellposeModel.eval(). That way anisotropy of 1 will have the same effect as it does now.

I'm testing this now