Output Refactor / Restructure

[mmcleod89]

To avoid outputs module becoming extremely cluttered and make some functions easier to write, we should move some output logic into the relevant modules e.g. pregnancy, sexual_behaviour etc.

• Can use output functions in each module, which take the output data-frame as an argument, to register relevant output data from each module.
• Useful outputs which are difficult to ascertain from the dataframe can simply be stored as member variables for that module as the simulation is running.
• This will also help to avoid the death distortion problem for our outputs.