Import errors when using `tasks._registration_utils.py` without `fractal-tasks` extra

[tcompa]

scMultiplex has this import: from fractal_tasks_core.tasks._registration_utils import create_well_acquisition_dict from the fractal_tasks_core.tasks subpackage.

Because it imports from fractal_tasks_core.tasks, we are expecting that the fractal-tasks-core[fractal-tasks] extra was included, but scMultiplex does not require it:

[options.extras_require]
fractal-tasks =
    anndata
    fractal-tasks-core==1.0.2

For this reason, scmultiplex imports from fractal_tasks_core.tasks may fail (e.g. due to the missing image_registration module, see #793 (comment).

How should we proceed?

1. #531 would clearly solve the problem, by making it explicit that you cannot import from tasks if you don't have the corresponding extra - but I don't think it's an easy solution.
2. We ask scMultiplex to install the fractal-tasks extra. This is quite bad, since they'll have to include a lot of heavy dependencies.
3. We move relevant functions (e.g. create_well_acquisition_dict) to the core-library, where no special imports are needed.
4. We mitigate the issue by deferring imports as much as possible, so that we reduce the risk of someone hitting a ModuleNotFoundError when importing some helper functions. An example of this solution is to move chi2_shift_out to a separate module, which scMultiplex will likely not import from.