Adds additional loss functions

[georgeyiasemis]

Adds:

• High frequency error norm (hfen) as loss and metric
• Signal-to-noise ratio (snr) loss
• SSIM 3d loss
• Tests

[codecov]

Codecov Report

Attention: Patch coverage is 88.73239% with 24 lines in your changes are missing coverage. Please review.

Project coverage is 84.05%. Comparing base (39e7680) to head (aec9327).

Files	Patch %	Lines
direct/functionals/hfen.py	81.08%	14 Missing ⚠️
direct/nn/mri_models.py	87.93%	7 Missing ⚠️
direct/functionals/snr.py	87.50%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #262      +/-   ##
==========================================
- Coverage   84.40%   84.05%   -0.36%     
==========================================
  Files          93       93              
  Lines        6645     6486     -159     
==========================================
- Hits         5609     5452     -157     
+ Misses       1036     1034       -2     

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[jonasteuwen]

Looks good, but some minor comments.

[jonasteuwen]

LGTM