ENH: N4 reorganization.

[ntustison]

Rename N4 functions: ANTsImage interface becomes n4_bias_field_correction

Add an ANTsImage interface to ANTsTorch's differentiable N4 so it can be called like ants.n4_bias_field_correction():

antstorch.n4_bias_field_correction(image, mask, shrink_factor=4, ...)

The interface converts ANTs images to tensors on the requested device, runs the tensor engine, and returns an ANTsImage in the input geometry.

Rename the tensor engine to avoid a name clash at the antstorch level (bspline_flows is imported after utilities and would otherwise shadow the new interface):

bspline_flows.n4_bias_field_correction
    -> bspline_flows.n4_bias_field_correction_tensor

N4BiasFieldCorrection (nn.Module) and module filenames are unchanged.

• utilities/n4_bias_field_correction.py: new ANTsImage interface
• utilities/init.py: export n4_bias_field_correction
• bspline_flows/{init,n4_bias_field_correction}.py: rename engine
• benchmark/data.py, docs tutorial, bspline_scattered_data docstring: use n4_bias_field_correction_tensor
• tools/benchmarks/compare_n4_bias_field_correction.py: call the ANTsImage interface; drop the local ants<->torch conversion helpers
• tests: update the bspline_flows N4 tests; add tests/utilities for the interface (geometry preservation, bias return, mask mismatch)