Integration of online reconstruction

[paquiteau]

This add support for online reconstruction paradigm (see also #129) TBA:

• [ ] Online Reconstructors
• [ ] Tests
  • [ ] Fourier Operators Test
  • [ ] Generator Test
  • [ ] Integration Test (run online reconstruction from beginning to end)

[chaithyagr]

Maybe, break down this PR to chunks, to prevent a large review overhead?

[codecov-commenter]

Codecov Report

Merging #130 (9bcf672) into master (f37f57c) will decrease coverage by 8.67%. The diff coverage is 21.15%.

@@            Coverage Diff             @@
##           master     #130      +/-   ##
==========================================
- Coverage   80.29%   71.62%   -8.68%     
==========================================
  Files          33       39       +6     
  Lines        1670     1864     +194     
==========================================
- Hits         1341     1335       -6     
- Misses        329      529     +200     

Flag	Coverage Δ
unittests	71.62% <21.15%> (-8.68%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mri/generators/__init__.py	0.00% <0.00%> (ø)
mri/generators/base.py	0.00% <0.00%> (ø)
mri/generators/cartesian.py	0.00% <0.00%> (ø)
mri/operators/fourier/online.py	0.00% <0.00%> (ø)
mri/operators/gradient/online.py	0.00% <0.00%> (ø)
mri/reconstructors/base.py	95.23% <ø> (ø)
mri/optimizers/utils/cost.py	57.57% <20.00%> (-4.50%)	:arrow_down:
mri/optimizers/base.py	45.28% <45.28%> (ø)
...i/reconstructors/utils/extract_sensitivity_maps.py	72.72% <58.82%> (-12.00%)	:arrow_down:
mri/optimizers/forward_backward.py	88.88% <80.00%> (-4.62%)	:arrow_down:
... and 2 more

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