Move global consensus transform reduction methods into a separate subpackage

## Current Behavior

The `itk_dreg.reduce_dfield` subpackage primarily implements a method to reduce multiple bounded transforms from subimage pairwise registrations into a single `itk.DisplacementFieldTransform`. But, the `itk_dreg.reduce_dfield.matrix_transform` subpackage also implements a method to estimate a global consensus or "average" of rigid transform results, returning a single rigid global transform instead of a displacement field.

## Requested Behavior

Methods for estimating a global consensus or "average", non-piecewise transform reduction result should be conceptually organized in a subpackage separate from `itk_dreg.reduce_dfield`.