Ensure attribute input connection is serialized, even for List/GroupAttributes.
Note: while connecting GroupAttributes is not yet supported, this will benefit
this type of attribute once it is.
* Deserialization: Replace the logic that defaulted the node type version to "0.0" when unspecified,
and assume that unspecified version on a node is targetting current node type version.
* Serialization: Only serialize node type versions for which a version info is available.
* Test suites:
* Add helper context manager to manually override the version of a given node type.
* Add new unit tests to cover version conflicts handling is various scenarios.
Add a new serializer class to manage partial graph serialization logic,
ensuring to remove link expressions on attributes refering to nodes
that are not in the subset of nodes to serialize.
Move the serialization logic to dedicated serializer classes.
Implement both `GraphSerializer` and `TemplateGraphSerializer`
to cover for the existing serialization use-cases.
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.
