Ecole observation features

[AntoinePrv]

Modularize feature extraction from scip::Model

Currently, feature extracted in NodeBipartite cannot be easily reused by other classes, or customized in the current class. On the other end, VarProxy... create confusion with SCIP variables.

• [x] Move the code extracting features from scip::Model to the observation namespace.
• [ ] Define Feature classes that
  • [ ] Are typed depending on the feature (scalar, categorical, on all variable, global...)
  • [ ] Contain directive for preprocessing: one-hot encode, static function to normalize...
  • [ ] Know (statically? dynamically?) if there need to be recomputed on every transition
• [ ] Take feature to extract as a vector / array of names (enum)
• [x] Leverage range library for maximum re-usability? and std::span
• [ ] Be performant, in particular maximize data locality and make compile-time compuattion when the features are given at compile-time.
• [ ] Determine where to put the function to extract row matrix
• [ ] Make it possible to reuse features to create new features, while efficiently caching the intermediate feature in the implementation