doc: Add "optional" tags on station plotting functions.
Updated colour maps in station plots.

* Switch to 'Paired' colour map from ColorBrewer, which is actually for
  categorical data.
* Make default colour in plots match Paired(lut=1) colour map.
* Specifically ask Matplotlib for a colour map with the length we need.
  This ensures that colours are correctly chosen from the colour map.
* Change non-default colourmap test to Set3, since hsv isn't really
  categorical.