Refactor darknet backbone as separate modules

[zhiqwang]

Fixes #41

• Refactor DarkNet
• Refactor BackboneWithPAN
• Refactor PathAggregationNetwork
• Modify checkpoint update scripts
• Cleanup hubconf and layers loading

[codecov]

Codecov Report

Merging #33 (424ddb2) into master (d3a4bff) will increase coverage by 0.14%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #33      +/-   ##
==========================================
+ Coverage   70.62%   70.77%   +0.14%     
==========================================
  Files           8        8              
  Lines         606      609       +3     
==========================================
+ Hits          428      431       +3     
  Misses        178      178              

Flag	Coverage Δ
unittests	70.77% <100.00%> (+0.14%)	⬆️

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Impacted Files	Coverage Δ
test/test_models.py	93.10% <100.00%> (+0.14%)	⬆️

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