Run distributed registration with `dask.distributed`

The distributed registration implementation defined in https://github.com/InsightSoftwareConsortium/itk-dreg/pull/6 uses `itk.Image`s in in method interfaces to describe image domains across voxel and physical spaces. Distributed registration with `dask.distributed` requires that inputs/outputs are serializable such that they may be transmitted between/across workers.

Unbuffered `itk.Image` representations are not yet serializable (pickleable), but will be in ITK v5.4rc3: https://github.com/InsightSoftwareConsortium/ITK/issues/4267

Once ITK v5.4rc3 is released we should revisit distributed registration in `itk-dreg`. At that time the `localcluster` unit test should be re-included in automated testing and used to verify distributed registration with `dask.distributed.LocalCluster` as a first step towards offsite distributed registration.