Add "Open in Colab" badge to README

[rgerkin]

Google Colab now allows arbitrary notebooks to be opened in an interactive Colab session, e.g. see the badge here

[rgerkin]

@ChihweiLHBird The goal here is for people to be able to open and run the notebooks right away without installing sciunit. We have been using Binder for this sort of thing, but it is has been unreliable. I would like to be able to open the same notebooks in Google Colab. The challenge is that Colab always starts from an environment that does not have sciunit installed. It is easy enough to add a !pip install sciunit at the top of any notebook, but I'm not sure that's best. The cell:

try:
  import google.colab
  IN_COLAB = True
except:
  IN_COLAB = False
if IN_COLAB:
  !pip install -q sciunit

I suppose could be added to the top of every notebook. Or maybe we should just assume that the user knows they will have to do !pip install sciunit at some point. Any thoughts?

[ChihweiLHBird]

Fixed in #152