Adds 3D vSHARP model

[georgeyiasemis]

• Implements 3D version of vSHARP model (Original paper: https://arxiv.org/abs/2309.09954)
• Implements 3D version of U-Net (extending https://github.com/NKI-AI/direct/blob/main/direct/nn/unet/unet_2d.py)

[codecov]

Codecov Report

Attention: Patch coverage is 96.49123% with 8 lines in your changes are missing coverage. Please review.

Project coverage is 84.76%. Comparing base (b660012) to head (725a119).

Files	Patch %	Lines
direct/nn/vsharp/vsharp.py	94.59%	4 Missing ⚠️
direct/nn/unet/unet_3d.py	98.33%	2 Missing ⚠️
direct/nn/unet/unet_2d.py	75.00%	1 Missing ⚠️
direct/nn/vsharp/vsharp_engine.py	96.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #273      +/-   ##
==========================================
+ Coverage   84.05%   84.76%   +0.70%     
==========================================
  Files          95       96       +1     
  Lines        6742     6963     +221     
==========================================
+ Hits         5667     5902     +235     
+ Misses       1075     1061      -14     

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[jonasteuwen]

Some remarks, mostly about documentation and variable choice and typing