Mimics aliby's.

WHY IS THIS CHANGE NEEDED?:
- these routines are useful outside of aliby

HOW DOES THE CHANGE SOLVE THE PROBLEM?:
- copied over from postprocessor.routines, but with reference to
  plottingabc fixed

WHAT SIDE EFFECTS DOES THIS CHANGE HAVE?:
- currently follows internal class-plotting function structure after
  seaborn -- can then convert them into pure functions for ease of use
- heatmap depends on standardscaler process.  decide to keep this for
  now because the colour scale depends on whether the data was scaled --
  i.e. there are lines to ensure that zero corresponds to the centre of the
  colour scale even if the min and max take different absolute values
- not sure how important plottingabc is, given that there's not much to
  inherit from it