v0.5.0

New features

• Add enum to NodeBipartite to describe feature columns.
• Add Khalil2016 observation function.
• SolvingTime rewards are now given in seconds (used to be miliseconds).
• Move instance generators from Python to C++
• Source build automatically download libraries if they are not found (except for SCIP) without Conan
• Ecole automatically builds with IPO when available and xsimd