Keep track of displacement fields along several registrations

- It seems scientists are doing registrations in an iterative way
- It could be nice to keep track of the resulting displacement fields and be able to compose them together
- Analyze if still give realistic results (Jacobian Matrix ?)
- Analyze if the compose displacement field applied to the input image is giving consistent results

The issue names no files, tests, or entry points. Start by locating the registration and displacement-field handling code, then determine how iterative fields could be stored and composed. Done would require a defined composition workflow plus evidence that Jacobian behavior and composed-field application produce consistent results.

Written by the indexing model from the issue text.