Draft: New ensemble framework

Description

Add the ensemble_t container for multitrajectory runs and adapt the multisystem framework, so that ensembles are correctly created by the factory.

News snippet

Add the ensemble_t container for multitrajectory runs and adapt the multisystem framework, so that ensembles are correctly created by the factory.

Checklist

• I have checked that my code follows the Octopus coding standards
• I have added tests for all the new features added in this request.

Closes #964