GitLab

Consistency & backward compatibility of OptHistory:

* GraphOptimiser and InitialGenerationBuilder now use initial_individuals instead of initial_graphs in favor of OptHistory consistency.
* Individual serialization: fitness that was saved as float in older histories is now converted to SingleObjFitness when loaded.
* OptHistory.load() is now able to get os.Pathlike objects, as its signature says.
* A new test for backward compatibility of OptHistory.load() is added.
* ParentOperator class is temporary imported at opt_history