Structure of the Python files

[marcenacp]

Use the same structure as JAX for the Python files:

ml_croissant
  |_ __init__.py  # Here main imports for the external API: atm only Dataset
  |_ _src
    |_ dataset.py  # Contains `Dataset`
    |_ record.py  # Contains `Record`
    |_ data_type.py
    |_ issues.py
    |_ operation_graph
      |_ graph.py  # Contains `ComputationGraph`
      |_ operations
        |_ download.py
        |_ read.py
        |_ ...  # Other operations
    |_ structure_graph
      |_ graph.py  # Contains `build_structure_graph`
      |_ nodes
        |_ __init__.py  # Contains the main class `Node`
        |_ metadata.py
        |_ field.py
        |_ file_object.py
        |_ file_set.py

All test files are right next to the file (e.g., issues.py is with issues_test.py).

[marcenacp]

Closed with:

• Refactor operations #96
• Simplify code and add developer-friendly documentation for nodes. #100
• Clean naming across files for operations #104