Implement ensemble framework

Description

Implement a basic class for multi trajectory ensembles. This is the first step, as we will address parallelism, injection of initial conditions, etc. in further merge requests.

This MR replaces !2478 (closed), which was obsoleted by the refactoring of the interaction factory.

News snippet

Implement a basic class for multi trajectory ensembles.

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 (closed)