Explore better ways for writing output

[timcallow]

See this comment. Probably nicer ways of outputting data from calculations. Note that not only the <x>_to_csv functions have potential for improvement but a lot of the writeoutput module is a bit hacked together. We might also want to introduce options to store various objects (e.g. using pickle), maybe bundled up together, for re-use in postprocessing / restarting calculations.