Add time to checkpoint file

[jhill1]

Describe the current issue
Time is not stored in any of the outputs (h5)

Describe the solution you'd like
Add the time variable to the h5 files, which is read in automatically when starting from such a checkpoint. When post-processing or checkpointing you would then no longer have to manually work out the time.

[dham]

The save_function method of CheckpointFile supports writing arbitrary timestepping metadata (such as time) to disk. This can be retrieved using the get_timestepping_history method. Does that do what you want?

[jhill1]

Possibly, yes. Let me add an issue on thetis that links to this to keep track and we can discuss further there.