Streamline multistage registration setup

## 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.