Reoganization
- BaseNode: is the base class for all nodes
- Node: is now dedicated to python nodes, with the implentation directly
in the process function
- CommandLineNode: dedicated to generate and run external command lines
* 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.
