Update operator fields to fit LinearOperators v2.13

[nHackel]

Together with LinearOperatorCollection 2.4 this should fix #272

[nHackel]

The operators themselves work, but the composition breaks at the moment because I removed type-piracy method:

Base.:∘(A::AbstractLinearOperator, B::AbstractLinearOperator) = ProdOp(A, B)

from LinearOperatorCollection. This currently results in certain recos constructing a function instead of a ProdOp. One solution would be to wrap the respective operators which are implemented as generic LinearOperators into custom wrappers akin to:

struct FooOp <: AbstractMRIOperator
    op::LinearOperator
    meta:: # ...
end

and then define composition for the AbstractMRIOperators. I might need to touch up an op or two from the collection as well.

Alternatively we could simply skip the composition operator in our reconstruction code. Time-wise I prefer the latter, but I'd really like to keep the composite op, since its just nice. I'll take another go at it tomorrow

[codecov]

Codecov Report

❌ Patch coverage is 82.05128% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.96%. Comparing base (bb43bd0) to head (ed31469).
⚠️ Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
MRIOperators/src/MapSliceOp.jl	0.00%	6 Missing ⚠️
MRIOperators/src/MRIOperators.jl	75.00%	1 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (bb43bd0) and HEAD (ed31469). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (bb43bd0)	HEAD (ed31469)
MRIFiles	1	0

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #273       +/-   ##
===========================================
- Coverage   75.02%   59.96%   -15.06%     
===========================================
  Files         103      101        -2     
  Lines        5414     5326       -88     
===========================================
- Hits         4062     3194      -868     
- Misses       1352     2132      +780     

Flag	Coverage Δ
MRIBase	50.19% <ø> (ø)
MRICoilSensitivities	93.07% <ø> (ø)
MRIFiles	?
MRIOperators	40.56% <20.51%> (-0.54%)	⬇️
MRIReco	42.37% <68.42%> (+0.05%)	⬆️
MRISampling	0.35% <ø> (ø)
MRISimulation	63.07% <ø> (ø)

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