Colab Compatibility

[mmcky]

For better compatibility with Google Colab we should

• migrate back from conda to pip for installing additional packages
• The jupyter notebook based contents links don't work on google colab pages

[mmcky]

I reviewed the first dot point

• conda in back_prop is required as plotly-orca is not available via pypi as it is hardware dependent library like graphviz. The note and comment to the code block is available on both html and on collab and explains the error. Also given that code-block executes the pip install first the notebook runs fine on collab. (See Why plotly-orca is unable to install via pip? plotly/orca#194)