Create model base class

[JPXKQX]

This PR creates a base class BaseAnemoiEncProcDecModel to encapsulate the common functionality between the different models:

• 1 encoder + 1 processor + 1 decoder (supported)
• 1 encoder + hierarchical processor + 1 decoder (supported in Feature/hierarchical graphs #37)
• N encoder + 1 processor + M decoder (WIP)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.85%. Comparing base (2ff0807) to head (268b7b2).

Additional details and impacted files

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##           develop      #79   +/-   ##
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  Coverage    99.85%   99.85%           
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  Files           23       23           
  Lines         1374     1374           
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  Hits          1372     1372           
  Misses           2        2           

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