Make "other" dimension more flexible

[ckolbPTB]

Suggestion by @fzimmermann89

Re the other dimension:

I would prefer if we allow in our code for this dimension to actually be more than 1 dimensnion.
So we dont split the KData into dynamics, ... by default, but our code works if it is split.

This would, as a start, mean replacing 'other' by '...' in all einops pattern, and counting dimensions from the back for swapaxes etc.

[ckolbPTB]

Make sure that always the correct terms (e.g. coils and not coil) are used.

Functions/Classes to double check

• mrpro/src/mrpro/algorithms/prewhiten_kspace.py

  Line 30 in 0730e06

  def prewhiten_kspace(kdata: KData, knoise: KNoise, scale_factor: float | torch.Tensor = 1.0) -> KData:

• mrpro/src/mrpro/operators/CartesianSamplingOp.py

  Line 26 in 0730e06

  class CartesianSamplingOp(LinearOperator):

• mrpro/src/mrpro/operators/DensityCompensationOp.py

  Line 25 in 0730e06

  class DensityCompensationOp(EinsumOp):

• mrpro/src/mrpro/operators/SensitivityOp.py

  Line 25 in 0730e06

  class SensitivityOp(LinearOperator):

• mrpro/src/mrpro/data/traj_calculators/KTrajectoryCartesian.py

  Line 48 in 0730e06

  ky = repeat(ky, 'other k2 k1-> other k2 k1 k0', k0=1)

• mrpro/src/mrpro/data/traj_calculators/KTrajectoryPulseq.py

  Line 73 in 0730e06

  def reshape_pulseq_traj(k_traj: torch.Tensor, encoding_size: int):

• mrpro/src/mrpro/data/IData.py

  Line 63 in 0730e06

  class IData(Data):

• mrpro/src/mrpro/data/KNoise.py

  Line 65 in 0730e06

  noise_data = rearrange(noise_data, 'other coils (k2 k1 k0)->other coils k2 k1 k0', k1=1, k2=1)

• mrpro/src/mrpro/data/KTrajectory.py

  Line 30 in 0730e06

  class KTrajectory(MoveDataMixin):

• mrpro/src/mrpro/data/KTrajectoryRawShape.py

  Line 30 in 0730e06

  class KTrajectoryRawShape(MoveDataMixin):

• mrpro/src/mrpro/data/QData.py

  Line 34 in 0730e06

  class QData(Data):

This probably wont work for flexible other dimensions but will be replaced at some point anyway:

• mrpro/src/mrpro/data/_kdata/KDataSelectMixin.py

  Line 31 in 0730e06

  def select_other_subset(

• mrpro/src/mrpro/data/_kdata/KDataSplitMixin.py

  Line 34 in 0730e06

  def _split_k2_or_k1_into_other(

[ckolbPTB]

Start by modifying the tests such that they fail for a current inflexible use of other (e.g. simply add an additional other singleton dimension) and then fix it.