Improve node serialization/deserialization to be able to recreate the exact same node in the graph when loading a meshroom project, even if the corresponding node's description has changed or does not exist anymore. This allows to recover already computed data on disk, without being impacted by changed uids. CompatibilityNode also provides an on-demand upgrade system to turn into a Node that meets the current node description (if possible).
* new abstract class BaseNode, base class for Node and CompatibiliyNode
* Node: serialize everything needed to spawn a CompatibilityNode with precomputed outputs: inputs, uids, parallelization settings, unresolved internal folders and outputs
* node_factory: handles node deserialization and compatibility issues to create either a Node or a CompatibilityNode
* add compatibility unit tests
