Add TPCDS(sqlContext) constructor for backwards-compatibility

This patch adds additional constructors to `TPCDS` to maintain backwards-compatibility with code which calls `new TPCDS(anExistingSqlContext)`. This constructor was removed in #47.

The motivation for backwards-compatibility here is to simplify the gradual roll-out of an updated spark-sql-perf library to some existing jobs which share the same notebook.

Author: Josh Rosen <rosenville@gmail.com>

Closes #52 from JoshRosen/backwards-compatible-tpcds-constructor.

src/main/scala/com/databricks/spark/sql/perf/tpcds/TPCDS.scala

@@ -19,19 +19,23 @@ package com.databricks.spark.sql.perf.tpcds
 import scala.collection.mutable
 
 import com.databricks.spark.sql.perf._
+import org.apache.spark.SparkContext
 import org.apache.spark.sql.SQLContext
 
 /**
  * TPC-DS benchmark's dataset.
+ *
  * @param sqlContext An existing SQLContext.
  */
-class TPCDS
-  extends Benchmark
+class TPCDS(@transient sqlContext: SQLContext)
+  extends Benchmark(sqlContext)
   with ImpalaKitQueries
   with SimpleQueries
   with Tpcds_1_4_Queries
   with Serializable {
 
+  def this() = this(SQLContext.getOrCreate(SparkContext.getOrCreate()))
+
   /*
   def setupBroadcast(skipTables: Seq[String] = Seq("store_sales", "customer")) = {
     val skipExpr = skipTables.map(t => !('tableName === t)).reduceLeft[Column](_ && _)