All data sources show up as "PhysicalRDD" in physical plan explain. It'd be better if we can show the name of the data source.

Without this patch:
```
== Physical Plan ==
NewAggregate with UnsafeHybridAggregationIterator ArrayBuffer(date#0, cat#1) ArrayBuffer((sum(CAST((CAST(count#2, IntegerType) + 1), LongType))2,mode=Final,isDistinct=false))
 Exchange hashpartitioning(date#0,cat#1)
  NewAggregate with UnsafeHybridAggregationIterator ArrayBuffer(date#0, cat#1) ArrayBuffer((sum(CAST((CAST(count#2, IntegerType) + 1), LongType))2,mode=Partial,isDistinct=false))
   PhysicalRDD [date#0,cat#1,count#2], MapPartitionsRDD[3] at
```

With this patch:
```
== Physical Plan ==
TungstenAggregate(key=[date#0,cat#1], value=[(sum(CAST((CAST(count#2, IntegerType) + 1), LongType)),mode=Final,isDistinct=false)]
 Exchange hashpartitioning(date#0,cat#1)
  TungstenAggregate(key=[date#0,cat#1], value=[(sum(CAST((CAST(count#2, IntegerType) + 1), LongType)),mode=Partial,isDistinct=false)]
   ConvertToUnsafe
    Scan ParquetRelation[file:/scratch/rxin/spark/sales4][date#0,cat#1,count#2]
```

Author: Reynold Xin <rxin@databricks.com>

Closes #8024 from rxin/SPARK-9733 and squashes the following commits:

811b90e [Reynold Xin] Fixed Python test case.
52cab77 [Reynold Xin] Cast.
eea9ccc [Reynold Xin] Fix test case.
fcecb22 [Reynold Xin] [SPARK-9733][SQL] Improve explain message for data source scan node.