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 #37)
• N encoder + 1 processor + M decoder (WIP)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.85%. Comparing base (8874571) to head (a3e279f). Report is 1 commits behind head on develop.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #79 +/- ## ======================================== Coverage 99.85% 99.85% ======================================== Files 23 23 Lines 1350 1350 ======================================== Hits 1348 1348 Misses 2 2 ```

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