Use Spark Accumulators to calculate Atlas Statistics

[MikeGost]

In the AtlasGenerator job, calculating statistics is a separate (optional) stage.

The idea is to replace this with a Spark Double Accumulator earlier in the flow. The Accumulator can still support the custom AtlasStatistics class and be optional.

This might improve the overall runtime of the statistics portion, as it would be done inline with Atlas creation. However, I don't have data to back up this assumption. Writing a task in case there is interest in streamlining this portion of the job.