Update checkpointing docs to include new functionality

[andrewcoughtrie]

PR Summary

Sci/Tech Reviewer: @stevemullerworth
Code Reviewer:

Updated the checkpointing documentation to include the new functionality around flexible checkpointing.

closes #23

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trac.log

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Code Review

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[stevemullerworth]

Couple of minor tweaks to reflect that there may be more than one dump.

[stevemullerworth]

Suggested change

	checkpoint dumps at multiple points within a model run. The checkpoint dump can be read in
	checkpoint dumps at multiple points within a model run. A checkpoint dump can be read in

[stevemullerworth]

Suggested change

	by a new integration of the model allowing further timesteps to be run. The dump
	by a new integration of the model allowing further timesteps to be run. Each dump