Pursue polyaffine transform reduction

[tbirdso]

Background

Polyaffine transforms present an approach to piecewise transform composition where several 3D centerpoints are paired with affine transforms and a function smooths transform output across a corresponding physical domain.

Similar to how itk_dreg.reduce_dfield implements the itk_dreg.base.registration_interface.ReduceResultsMethod virtual interface, we could implement a reduction method to reduce several piecewise affine inputs into a single polyaffine transform result. A polyaffine transform may require significantly less storage than a displacement field to describe transformations over a given region.

Feature Request

1. Recommended: Create a new itk-dreg-polyaffine project repository for the extension.
2. Have itk_dreg_polyaffine depend on itk_dreg in its pyproject.toml
3. Implement PolyaffineReduceResultsMethod extending ReduceResultsMethod
4. Distribute itk-dreg-polyaffine on PyPI with example usage and testing