Clean up demo notebooks

I think it would be a good idea to clean up the notebooks in demo_notebooks a little bit, especially because we are planning to include them on the website. A few suggestions:

• Removing the commented out code. Let's keep these notebooks tidy!
• Moving some of the class and function definitions to modules to make the notebooks more compact.
• Including a short introduction at the beginning of each notebook to summarize what will be done.
• Possibly including Figure 1 and 2 from the Supplementary Information (which show the network architectures) of the ClimSim paper at the beginning of the two notebooks cnn_example.ipynb and mlp_example.ipynb.
• Including a few more figures on the way (if possible), and make the figures look a little bit nicer. A website / notebooks with nice figures will be read by more people - no joke!

I am happy to help with this, if other people agree with the suggestions above. @jerrylin96 @cisaacstern?

Start by reviewing the notebooks in demo_notebooks, especially cnn_example.ipynb and mlp_example.ipynb, and compare their current contents with the issue checklist. Done means commented-out code is removed, suitable definitions are moved to modules, each notebook has an introduction, and the requested architecture and additional figures are included where appropriate.