refactor exporting

[drbenvincent]

We have these repeated code elements spread throughout high and low-level functions and classes

[drbenvincent]

So far we've:

• extracted some of the export code upward from low level areas to higher level areas. This is good because it has made the lower level functions focus on their thing, and separate from the exporting responsibility. Also means parsing less arguments down the tree
• removed some duplication

Still need to extract export code upwards from:

• [ ] Parametric.plotAllTriPlots
• [ ] NonParametric.psychometric_plots
• [ ] Model.plotAllExperimentFigures
• [x] PosteriorPrediction.plot

All of these iterate over multiple experiment files and so produce (and export) multiple plots.

[drbenvincent]

I think this is enough tidying up