Fix operator chaining

[fzimmermann89]

Two spots I missed when we switched to operators returning tuples:
In the @ chain, we need to unpack the return of the inner operator when calling the outer operator.

Mypy did not catch that as the types on the general operator are not bound to Tensor.

Edit: A couple more issues....
We should maybe consider running mypy on the tests as well...

(Maybe https://stackoverflow.com/questions/78013162/typehint-function-args-tupleargs-with-constrain-on-the-args could at some point help here)

[github-actions]

Coverage Report

File	Stmts	Miss	Cover	Missing
src/mrpro/algorithms
_adam.py	13	8	38%	93–103
_lbfgs.py	14	8	43%	68, 85–94
_remove_readout_os.py	30	2	93%	46, 48
src/mrpro/data
_AccelerationFactor.py	13	13	0%	15–37
_AcqInfo.py	79	2	97%	100, 135
_Data.py	15	2	87%	58, 73
_DcfData.py	82	4	95%	34, 75, 101, 189
_IData.py	57	7	88%	44, 49, 109, 114, 122, 128, 132
_IHeader.py	58	2	97%	86, 113
_KData.py	111	12	89%	103–104, 200, 202, 232, 237–238, 282, 284, 288, 318, 334
_KHeader.py	125	9	93%	34, 82, 120, 171, 182, 189–190, 193, 200
_KNoise.py	21	9	57%	56–69
_KTrajectory.py	63	2	97%	228, 248
_SpatialDimension.py	39	5	87%	76, 79, 86–88
_TrajectoryDescription.py	14	1	93%	37
src/mrpro/data/traj_calculators
_KTrajectoryCalculator.py	26	3	88%	37, 63, 65
_KTrajectoryIsmrmrd.py	15	2	87%	55, 62
_KTrajectoryPulseq.py	33	1	97%	72
src/mrpro/operators
_CartesianSamplingOp.py	48	9	81%	58–59, 64–65, 70–71, 95, 98, 122
_ConstraintsOp.py	58	2	97%	34, 36
_LinearOperator.py	49	4	92%	37, 90, 106, 111
_Operator.py	45	1	98%	36
_ZeroPadOp.py	15	1	93%	45
src/mrpro/utils
_rgetattr.py	3	1	67%	20
_zero_pad_or_crop.py	30	6	80%	37, 41, 64, 67, 70, 73
TOTAL	1729	116	93%

Tests	Skipped	Failures	Errors	Time
189	0 💤	0 ❌	0 🔥	35.378s ⏱️

[fzimmermann89]

TODO: Check if mypy likes the tests

[schuenke]

The need to use typing.cast() does not necessarily make the code easier to read, but unfortunately it seems to be unavoidable.

The rest looks fine and I checked already that everything is compatible with the ruff settings from #193