Enhancement for example venus_cube_mask.py

[robmarkcole]

To make the plot display when run from the command line, it's necessary to include:

import matplotlib.pyplot as plt

# at end
plt.show()

[robmarkcole]

Also true for venus_cube.py

[nkarasiak]

Thanks ! I kept only this venus example in order to build the documentation fastly : https://geosys.github.io/earthdaily-python-client/_auto_examples/venus_cube_mask.html#sphx-glr-auto-examples-venus-cube-mask-py