support "split by" property

Currently the way we [group](https://cedar-v1.surge.sh/?type=bar-grouped) or [stack](https://cedar-v1.surge.sh/?type=bar-stacked) values is by executing multiple queries and joining them. There is another way though. We could instead pass multiple fields to the `groupByFieldsForStatistics` and make a single query and then convert the response into the same format that is currently the output of the `join` operation.

This is how Ops Dahsboard does their split by queries and I feel like it should be the default way that cedar does them. For example, would be _much_ nicer if the [radar example](https://cedar-v1.surge.sh/?type=radar) had 1 query instead of 9.

In order to do this, we'd likely need to add something like `splitBy` or `splitByFields` to the definition so we know how to process the query results. We should look at the Ops Dahsboard API (and maybe even source code 😈) for "inspiration."