[SPARK-19634][ML] Multivariate summarizer - dataframes API

## What changes were proposed in this pull request?

This patch adds the DataFrames API to the multivariate summarizer (mean, variance, etc.). In addition to all the features of MultivariateOnlineSummarizer, it also allows the user to select a subset of the metrics.

## How was this patch tested?

Testcases added.

## Performance
Resolve several performance issues in apache#17419, further optimization pending on SQL team's work. One of the SQL layer performance issue related to these feature has been resolved in apache#18712, thanks liancheng and cloud-fan

### Performance data

(test on my laptop, use 2 partitions. tries out = 20, warm up = 10)

The unit of test results is records/milliseconds (higher is better)

Vector size/records number | 1/10000000 | 10/1000000 | 100/1000000 | 1000/100000 | 10000/10000
----|------|----|---|----|----
Dataframe | 15149  | 7441 | 2118 | 224 | 21
RDD from Dataframe | 4992  | 4440 | 2328 | 320 | 33
raw RDD | 53931  | 20683 | 3966 | 528 | 53

Author: WeichenXu <[email protected]>

Closes apache#18798 from WeichenXu123/SPARK-19634-dataframe-summarizer.

Author: WeichenXu123

mllib/src/main/scala/org/apache/spark/ml/linalg/VectorUDT.scala

@@ -27,17 +27,7 @@ import org.apache.spark.sql.types._
  */
 private[spark] class VectorUDT extends UserDefinedType[Vector] {
 
-  override def sqlType: StructType = {
-    // type: 0 = sparse, 1 = dense
-    // We only use "values" for dense vectors, and "size", "indices", and "values" for sparse
-    // vectors. The "values" field is nullable because we might want to add binary vectors later,
-    // which uses "size" and "indices", but not "values".
-    StructType(Seq(
-      StructField("type", ByteType, nullable = false),
-      StructField("size", IntegerType, nullable = true),
-      StructField("indices", ArrayType(IntegerType, containsNull = false), nullable = true),
-      StructField("values", ArrayType(DoubleType, containsNull = false), nullable = true)))
-  }
+  override final def sqlType: StructType = _sqlType
 
   override def serialize(obj: Vector): InternalRow = {
     obj match {
@@ -94,4 +84,16 @@ private[spark] class VectorUDT extends UserDefinedType[Vector] {
   override def typeName: String = "vector"
 
   private[spark] override def asNullable: VectorUDT = this
+
+  private[this] val _sqlType = {
+    // type: 0 = sparse, 1 = dense
+    // We only use "values" for dense vectors, and "size", "indices", and "values" for sparse
+    // vectors. The "values" field is nullable because we might want to add binary vectors later,
+    // which uses "size" and "indices", but not "values".
+    StructType(Seq(
+      StructField("type", ByteType, nullable = false),
+      StructField("size", IntegerType, nullable = true),
+      StructField("indices", ArrayType(IntegerType, containsNull = false), nullable = true),
+      StructField("values", ArrayType(DoubleType, containsNull = false), nullable = true)))
+  }
 }