TST: Test that the estimator iterates correctly over the volumes

[jhlegarreta]

Test that the estimator iterates correctly over the volumes: check that for a dummy DWI model and the different iterators, the transform written by the estimator takes the index dictated by the iterator at issue.

[jhlegarreta]

Demonstrates a bug for the "bvalue" iterator: since it is a kwarg for the iterator at issue, and even if we would be providing it, the estimator's run method requires some further logic to distinguish the kwargs it requires for other things,

nifreeze/src/nifreeze/estimator.py

Lines 126 to 128 in 431ce51

	# Prepare iterator
	iterfunc = getattr(iterators, f"{self._strategy}_iterator")
	index_iter = iterfunc(len(dataset), seed=kwargs.get("seed", None))

e.g.

seed = kwargs.pop("seed", None)
index_iter = iterfunc(len(dataset), seed=seed, **kwargs)

Will not not work because kwargs is used further down

nifreeze/src/nifreeze/estimator.py

Lines 166 to 167 in 431ce51

	kwargs["num_threads"] = n_threads
	kwargs = self._align_kwargs | kwargs

So this will become even worse after #238 if the bug does not get fixed.