Checkpoint-schedules

- Update to published paper that proposes the `MixedCheckPointSchedule`.
- `MixedCheckPointSchedule` refactoring.

- Inclusion of an example of `checkpoint_schedules` usage in an adjoint-based gradient problem.

- Contributions of all the authors are more explicit by introducing an author contribution section in the paper, and by adding copyright headers with accurate authorship information to all of the source files.

**_For publication in Zenodo_**

_checkpointing_schedules_ provides schedules for step-based incremental checkpointing of the adjoints to computer models. The schedules contain instructions indicating the sequence of forward and adjoint steps to be executed and the data storage and retrieval to be performed. These instructions are independent of the model implementation, which enables the model authors to switch between checkpointing algorithms without recoding. Conversely, _checkpointing_schedules_ provides developers of checkpointing algorithms with a direct mechanism to convey their work to model authors.

The complete documentation for _checkpoint_schedules_ is available at https://www.firedrakeproject.org/checkpoint_schedules/

First release.