fixed colorbar issue

This PR modifies  the behaviour of the ```iris.plot``` functions so that rather than calling ```ax.draw_func``` they call ```plt.draw_func```. This is currently the case for non-map type plots, but anything going via cartopy via ```_map_common``` moves from the ```plt``` functions to geoaxes methods. This has an undesirable side effect that means subsequent calls to ```plt.colorbar()``` break when using ```iris.plot.contour```, ```iris.plot.pcolormesh``` etc. rather than their ```plt``` equivalents.

Fixes #1111.