Investigate approaches for successive ITKElastix deformable registration stages

## Current Behavior

ITKElastix accepts a generic `ExternalInitialTransform` to provide a generic `itk.Transform` as the initial forward transform to use. However, ITKElastix fails to run registration when `ExternalInitialTransform` cannot be translated to a known Elastix type to provide for advanced computation of derivatives, Jacobians, etc.

ITKElastix may also accept an initial Elastix transform object composed of Elastix registration results. It is not immediately obvious how to generate a valid Elastix transform from a reduced ITK transform result from a previous `itk-dreg` registration stage.

## Steps to Investigate

1. Understand how to map `itk.Transform`s to ITKElastix transform parameter objects
2. Determine whether ITKElastix transform parameter objects are serializable (see https://github.com/InsightSoftwareConsortium/itk-dreg/issues/16)
3. Determine how we can convert the transform result of a `ReduceResultsMethod` to an Elastix transform parameter object and whether limitations exist, such as the type of reduced results that can be converted, etc
4. Update the `itk_dreg.elastix` registration interface to accept an input Elastix transform parameter object
5. Create an example demonstrating multiple stages of deformable registration over successively higher image resolutions with ITKElastix