Redesign of Parareal driver to consider windowing

Issue/feature addressed

The Parareal driver has been redesigned in order to allows "windowing", i.e. splitting the time-integration into several time interval. This feature can be useful if a very large number of timesteps is required. In addition, the Parareal driver has been redesigned in prevision to the introduction of the PFASST driver. For the purpose of considering various Parareal-like parallel-in-time algorithms, a base class DriverParallelInTime has been created, while the DriverParareal has been refactored as a derived class of the former. Additionally, the DriverParareal class has been refactored for better readability.

Tests

Suggested reviewers

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Notes

Checklist

• Functions and classes, or changes to them, are documented.
• [ ] User guide/documentation is updated.
• Changelog is updated.
• [ ] Suitable tests added for new functionality.
• Contributed code is correctly formatted. (See the contributing guidelines).
• License added to any new files.
• No extraneous files have been added (e.g. compiler output or test data files).