Remove unambiguous complex assertions

[georgeyiasemis]

Main changes:

• assert_complex(shape, complex_last) --> assert_complex(shape, complex_axis, complex_last=None)
• modulus_if_complex removed
• modulus need to specify axis
• MRIModelEngine cropper expect the modulus of predicted data

[codecov[bot]]

Codecov Report

Merging #194 (7fa97de) into main (6308635) will decrease coverage by 0.75%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #194      +/-   ##
==========================================
- Coverage   76.53%   75.78%   -0.76%     
==========================================
  Files          71       71              
  Lines        5430     5100     -330     
==========================================
- Hits         4156     3865     -291     
+ Misses       1274     1235      -39     

Impacted Files	Coverage Δ
direct/engine.py	75.29% <ø> (ø)
direct/data/transforms.py	94.38% <100.00%> (-0.46%)	:arrow_down:
direct/nn/mri_models.py	93.67% <100.00%> (-2.31%)	:arrow_down:
direct/nn/rim/rim_engine.py	91.80% <100.00%> (-0.34%)	:arrow_down:
direct/utils/__init__.py	65.30% <100.00%> (-2.53%)	:arrow_down:
direct/utils/asserts.py	80.95% <100.00%> (+2.69%)	:arrow_up:
... and 1 more

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