🆕 Add Cerberus

[measty]

This PR adds Cerberus model architecture and pretrained models, so it can be used within the newly-efficient tiatoolbox engines.

Still a draft as needs tidying up a bit and uploading weights to huggingface.

Example output:

[codecov]

Codecov Report

❌ Patch coverage is 95.89041% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.72%. Comparing base (9d5abca) to head (a6bed60).

Files with missing lines	Patch %	Lines
...iatoolbox/models/architecture/cerberus/postproc.py	88.29%	10 Missing and 1 partial ⚠️
tiatoolbox/models/architecture/cerberus/model.py	97.74%	1 Missing and 2 partials ⚠️
...iatoolbox/models/architecture/cerberus/net_desc.py	94.91%	1 Missing and 2 partials ⚠️
tiatoolbox/annotation/utils.py	97.67%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1060      +/-   ##
===========================================
- Coverage    99.88%   99.72%   -0.17%     
===========================================
  Files           85       93       +8     
  Lines        11623    12060     +437     
  Branches      1524     1570      +46     
===========================================
+ Hits         11610    12027     +417     
- Misses           7       21      +14     
- Partials         6       12       +6     

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