[core] Introducing new graphIO module

Move Graph.IO internal class to its own module, and rename it to `GraphIO`.
This avoid nested classes within the core Graph class, and starts decoupling
the management of graph's IO from the logic of the graph itself.

meshroom/core/graphIO.py

@@ -0,0 +1,56 @@
+from enum import Enum
+from typing import Union
+
+from meshroom.core import Version
+
+
+class GraphIO:
+    """Centralize Graph file keys and IO version."""
+
+    __version__ = "2.0"
+
+    class Keys(object):
+        """File Keys."""
+
+        # Doesn't inherit enum to simplify usage (GraphIO.Keys.XX, without .value)
+        Header = "header"
+        NodesVersions = "nodesVersions"
+        ReleaseVersion = "releaseVersion"
+        FileVersion = "fileVersion"
+        Graph = "graph"
+
+    class Features(Enum):
+        """File Features."""
+
+        Graph = "graph"
+        Header = "header"
+        NodesVersions = "nodesVersions"
+        PrecomputedOutputs = "precomputedOutputs"
+        NodesPositions = "nodesPositions"
+
+    @staticmethod
+    def getFeaturesForVersion(fileVersion: Union[str, Version]) -> tuple["GraphIO.Features",...]:
+        """Return the list of supported features based on a file version.
+
+        Args:
+            fileVersion (str, Version): the file version
+
+        Returns:
+            tuple of GraphIO.Features: the list of supported features
+        """
+        if isinstance(fileVersion, str):
+            fileVersion = Version(fileVersion)
+
+        features = [GraphIO.Features.Graph]
+        if fileVersion >= Version("1.0"):
+            features += [
+                GraphIO.Features.Header,
+                GraphIO.Features.NodesVersions,
+                GraphIO.Features.PrecomputedOutputs,
+            ]
+
+        if fileVersion >= Version("1.1"):
+            features += [GraphIO.Features.NodesPositions]
+
+        return tuple(features)
+