A couple of changes to centroid_diff_{x,y}.png plots.
A couple of changes to centroid_diff_{x,y}.png plots.

* Centred heat plot range so that 0.0 always maps to the same colour (green)
* Optional expert parameter centroid_diff_max sets the value at the extremes
  of the colour bar, so it is easier to compare multiple plots
dials/dials,dials/dials,dials/dials,dials/dials,dials/dials