`draw` no longer needed for mpu.colorbar

[mathause]

The newest version of mplotutils makes the call to plt.draw() or f.canvas.draw() redundant. We could adjust the instructions. But maybe wait until this is at least in one IAC python environment.

f, ax = plt.subplots(...)
...
mpu.colorbar(h, ax)

-f.canvas.draw()

plt.savefig(...)

Relevant exercises:

• ex2_4_colorbars.ipynb
• ex2_5_stippling.ipynb
• ex5_2_multiple_regional_maps_M_Hauser.ipynb