Feature/datatables view enhance

[wardi]

Here's my version of the histogram API. I'm returning the buckets and edges in the "fields" value instead of as a separate dict.

I haven't updated the frond-end to match, but you can see the expected values in the test provided.

We'll need a helper function to turn edges and bucket array values into histogram bar heights and widths. With discrete values like ints or dates some bars will be different widths, and the heights need to be scaled by the relative widths for accuracy.

Other future work:

• buckets should be calculated based on the filters of only the other columns so that users can adjust the histogram brush controls outside their current values
• redis cache for bucket calculations, only cache when no filters applied, delete individual cache values on any change to the datastore table

[wardi]

oops, pushed to the wrong branch and closed this accidentally