Donwload model only when needed

[RonanMorgan]

This code is :

• creating a meadata file with the hash of the model if this doesn't exist
• if already existing , it compares the hash downloaded and the hash of the file
• if the file doesn't exist or the hash are different it will download the new model

TODO : we still need to create a new path name in huggingface space in order to have a "neutral name" (like pyronear/model.onnx) and not having to change the value in the code each time @MateoLostanlen

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.91%. Comparing base (c18de6b) to head (0acd8d5).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #200      +/-   ##
===========================================
+ Coverage    87.84%   88.91%   +1.07%     
===========================================
  Files            6        6              
  Lines          362      397      +35     
===========================================
+ Hits           318      353      +35     
  Misses          44       44              

Flag	Coverage Δ
unittests	88.91% <100.00%> (+1.07%)	⬆️

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[MateoLostanlen]

Hi Ronan, this looks togood to me. Just can you change the model path and hash based on PR #202