Test notebooks in colab and add launch in colab button

[stevehadd]

In order to make it easier for a general random person to try out pyearthtools (i.e. without having access to a "preapproved" platform), we should test running on google colab and add a "Launch in colab" button to all the notrbooks, and dpossibly the repo readme.

docs oin hjow to add launch in colab button:

• https://jupyterbook.org/v1/interactive/launchbuttons.html
• https://www.youtube.com/watch?v=RoGZIbwzG5w