Pursue polyaffine transform reduction

## 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