Contour options don't play nicely with each other.

[bnlawrence]

The setup options for contours do not play nicely.

Consider the following sequence of events:

1. Default contour map. Plot.
2. Change number of contour intervals to 6 (setup/contours). Plot.
3. Change to user set contour levels (e.g min, max, step in setup/contour_levels). Plot.

• at this point you get six contour intervals rather than than (max-min)/step +1 ... and the maximum chosen is not obvious, although the minimum is honoured.

4. You can go back and reset the number of contour intervals, and then plot, and you get the user set contour levels honoured.

These menus should be combined in some way so that odd results are not obtained.