added 2d_contourf plot_type for filled contours...
not yet fully tested
merged mandli/mandli-add-contourf ideas into rjleveque/contourf...
Two options: can set plotitem.fill_cmap and then _cmin and _cmax for a colormap or can set plotitem.fill_colors with a list of colors to use
Merge pull request #81 from rjleveque/contourf

Add contourf `plot_type`