Added plot_atoms() method.

[dignojrteogalbo]

Closes #132, #131

Discussion

Added the plot_atoms() method which takes a Cube object. You can override the existing dictionaries for atom size and atom color.

Testing

• Run ./build.sh to create the virtual environment. (No new dependencies have been installed).
• Open GoVizzy.ipynb using the Python kernel from the virtual environment
• Execute the import cell. (Verify that meshes is imported from GoVizzy).
  
• Execute the cell performing plot_atoms().
  
• Verify the output widget contains both a mesh (sphere surface) and the volume.
  

[BraydenThompson]

Looks great! Thanks for figuring this out!

[ryliewalsh]

Works and looks good!