Collect groupby examples

[deppen8]

For help with #24, we need to better understand the range of groupby uses. We should collect some real-world uses of groupby to better understand whether a linting pattern will raise too many false positives.

[simchuck]

I am planning to look into the github dataset on Google BigQuery. If I can make sense of it, it should hopefully be straightforward to run a query to search for uses of groupby across many repos.