Added Pivot method to RelationalGroupedDataset

[moredatapls]

Added support for RelationalGroupedDataset.Pivot (excluding the overloads with values).

Fixes #591

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[dnfadmin]

All CLA requirements met.

[imback82]

Thanks @moredatapls! Is this ready for PR? I see the PR as "Draft".

[moredatapls]

@imback82 It's still a draft because I wanted to run the tests locally but I'm getting a NullReferenceException when I try to run them, see the strack trace below. I probably didn't configure my environment correctly, is there any guide that describes the local setup? Built the jars and the worker and configured the DOTNET_WORKER_DIR to point to the publish directory.

System.NullReferenceException: Object reference not set to an instance of an object.
   at Microsoft.Spark.E2ETest.SparkFixture.<>c__DisplayClass10_0.<.ctor>b__0(Object sender, DataReceivedEventArgs arguments) in /Users/helge.bruegner/Projects/spark/src/csharp/Microsoft.Spark.E2ETest/SparkFixture.cs:line 78
   at System.Diagnostics.Process.OutputReadNotifyUser(String data)
   at System.Diagnostics.AsyncStreamReader.FlushMessageQueue(Boolean rethrowInNewThread)
--- End of stack trace from previous location where exception was thrown ---
   at System.Diagnostics.AsyncStreamReader.<>c.<FlushMessageQueue>b__18_0(Object edi)
   at System.Threading.QueueUserWorkItemCallbackDefaultContext.Execute()
   at System.Threading.ThreadPoolWorkQueue.Dispatch()
   at System.Threading._ThreadPoolWaitCallback.PerformWaitCallback()

[suhsteve]

A few minor comments, but LGTM. Thanks @moredatapls !

[Niharikadutta]

LGTM as well, thanks @moredatapls !

[imback82]

LGTM except for nit comments

[imback82]

LGTM, thanks @moredatapls!