De-duplicate Pydantic models in lib_channels.py and lib_ngff.py

[tcompa]

Likely the solution will be to import models from lib_ngff.py into lib_channels.py, and then add/override methods or attributes (e.g. as in Channel and FractalChannel(Channel)). The goal is clearly to avoid having two definitions of the same model.

Also: functions like get_channel_from_image_zarr should probably become methods of the appropriate object (NgffImageMeta, if they are sufficiently general).

Ideally this should not lead to any change of existing JSON Schemas.