Use Spark job grouping to distinguish steps of the machine learning flow

[nicodv]

Related issues
N/A

Describe the proposed solution
Leverages Spark's ability to set a "job group" ID to distinguish certain steps of the machine learning. Examples: data IO, model IO, feature engineering, cross-validation.

OpSparkListener is extended to capture which job group is currently active. Also, the new OpStep enum's entry names automatically show up in the Spark UI so that the function of stages can be more easily interpreted.

To this end:

• An OpStep enum is introduced
• A utility is introduced to set the job group for a code block, or indefinitely

Describe alternatives you've considered
Because a main goal is to get the current step into the real-time SparkListener framework, the latter's ability to get hold of the Spark job group was an easy way to accomplish this.
Considered but not feasible:

• Spark listeners per step by adding/removing Spark listeners to/from the listener bus. More complex than using the job grouping, without added benefit.
• Spark's local properties, but it seems they are not shared with Spark listeners

Also considered were the addition of other steps, such as "sanity checker", "scoring" or "metrics". However, these are not included here as this would:

• Needed to be done in places inconsistent with other steps (most of the grouping is contained to OpWorkflow/OpWorkflowModel)
• Lead to additional overhead/complexity, e.g. because scoring and metrics calculation is part of a single Spark stage and therefore could not be split up by Spark job group.

Additional context
The extension to OpSparkListener allows for more advanced handling of the metrics that are collected by it, e.g. the metrics can be grouped by OpStep.

[codecov]

Codecov Report

Merging #467 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #467      +/-   ##
=========================================
+ Coverage   86.99%     87%   +0.01%     
=========================================
  Files         344     345       +1     
  Lines       11576   11575       -1     
  Branches      370     593     +223     
=========================================
+ Hits        10070   10071       +1     
+ Misses       1506    1504       -2

Impacted Files	Coverage Δ
...main/scala/com/salesforce/op/OpWorkflowModel.scala	93.9% <100%> (-0.15%)	⬇️
.../src/main/scala/com/salesforce/op/OpWorkflow.scala	88.11% <100%> (-0.85%)	⬇️
...sforce/op/stages/impl/selector/ModelSelector.scala	98.36% <100%> (+0.17%)	⬆️
...a/com/salesforce/op/utils/spark/JobGroupUtil.scala	100% <100%> (ø)
.../main/scala/com/salesforce/op/OpWorkflowCore.scala	95.45% <100%> (ø)	⬆️
...om/salesforce/op/utils/spark/OpSparkListener.scala	98.63% <100%> (+0.02%)	⬆️
...es/src/main/scala/com/salesforce/op/OpParams.scala	89.79% <0%> (+4.08%)	⬆️

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[tovbinm]

Great addition!!

[gerashegalov]

LGTM