Streamline multistage registration setup

[tbirdso]

Current Behavior

A single call to register_images reflects a single registration stage with the following map/reduce approach:

1. Subdivide fixed image into subimage tasks
2. In each task, fetch fixed and moving subimages, then register them together
3. Reduce registration results into a single descriptive transform

Note that one call to register_images reflects distributed registration for a single set of input parameters, such as the fixed image resolution, moving image resolution, and initial transform. A subimage registration may perform registration with multiple successive transforms. For instance, ITKElastix could register a set of rigid -> affine -> deformable transforms between two subimages in a single distributed registration stage.

It can be tedious to set up multiple registration stages with multiple calls to register_images. We should explore how the interface may be streamlined to improve the user experience without sacrificing the granularity of the present itk-dreg interface.

Proposed resolution

1. Create a new submodule itk_dreg.multistage
2. Define a multistage registration configuration object or schema that provides parameters for each register_images call:

• The fixed image source (with what resolution)
• The moving image source (with what resolution)
• The registration method to use
• The reduction method to use
• Overlap factors
• Chunk size

3. Define a register_multistage method that receives a registration configuration and successively calls register_images. On each call, provide a new itk.CompositeTransform initial transform that composes the ordered transform results of previous stages.
4. Add unit tests and create an example notebook demonstrating the streamlined interface.