Add FeaturePyramidModel

[ClementWalter]

This PR intends to add a FeaturePyramidModel model builder for training single-shot detectors like YOLO or SSD.

[codecov]

Codecov Report

Merging #10 (2d768a0) into master (15de9e5) will decrease coverage by 3.61%.
The diff coverage is 25.49%.

❗ Current head 2d768a0 differs from pull request most recent head 5712a48. Consider uploading reports for the commit 5712a48 to get more accurate results

@@            Coverage Diff             @@
##           master      #10      +/-   ##
==========================================
- Coverage   65.93%   62.32%   -3.62%     
==========================================
  Files          44       50       +6     
  Lines         963     1059      +96     
==========================================
+ Hits          635      660      +25     
- Misses        328      399      +71     

Impacted Files	Coverage Δ
keras_fsl/losses/yolo_loss.py	0.00% <0.00%> (ø)
keras_fsl/models/layers/__init__.py	0.00% <0.00%> (ø)
keras_fsl/utils/training.py	0.00% <0.00%> (ø)
keras_fsl/models/feature_pyramid_net.py	30.76% <30.76%> (ø)
keras_fsl/models/activations/yolo_coordinates.py	50.00% <50.00%> (ø)
keras_fsl/models/encoders/darknet.py	31.03% <50.00%> (+3.76%)	⬆️
keras_fsl/models/activations/yolo_box.py	80.00% <80.00%> (ø)
keras_fsl/models/__init__.py	100.00% <100.00%> (ø)
keras_fsl/models/activations/__init__.py	100.00% <100.00%> (ø)

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