Replace nipype ants interfaces with antspy?

[arokem]

I've recently had the pleasure of working with antspy. It's fast and the API is both straightforward and flexible. I wonder whether we should use this directly in the estimator module, instead of the nipype interface. Benefits include the fact that it's a pip install, so we would remove a pretty heavy dependency (and for many users, the docker dependency!).

[arokem]

This is a good place to start: https://github.com/nipreps/eddymotion/blob/main/src/eddymotion/estimator.py#L119