WIP: Add summary statistics to the dataresource
- [ ] closes 
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Following the discussion in https://github.com/nteract/nteract/issues/3177, this adds summary statistics for folks using the dataresource. Our main concern is how this might affect performance.

I wouldn't have put this up before the tests are passing, but as @rgbkrk and I are currently attending a Numfocus event, we hoped we might catch up with @jreback and/or others in person if anyone is attending to discuss and wanted to get the ball rolling 🙂. 

cc @TomAugspurger 