introduce new colorbar.draw method

- colorbar.draw is now invoked alongside the other
  'margin-pusher' components (e.g. legend, updatemenus ...)
- colorbar.draw draws ALL colorbar on a graph, by looping
  over the data to find how many colorbars there are
- use d3-update pattern to remove old colorbars
- colorbar.draw handles updates, simplifying doColorbars subroutine
- contour.colorbar defines a 'calc' method to generate its special
  colorbar options

Author: etpinard