SPARK-3568 [mllib] add ranking metrics

Add common metrics for ranking algorithms (http://www-nlp.stanford.edu/IR-book/), including:
 - Mean Average Precision
 - Precisionn: top-n precision
 - Discounted cumulative gain (DCG) and NDCG

The following methods and the corresponding tests are implemented:

```
class RankingMetrics[T](predictionAndLabels: RDD[(Array[T], Array[T])]) {
  /* Returns the precsionk for each query */
  lazy val precAtK: RDD[Array[Double]]

  /**
   * param k the position to compute the truncated precision
   * return the average precision at the first k ranking positions
   */
  def precision(k: Int): Double

  /* Returns the average precision for each query */
  lazy val avePrec: RDD[Double]

  /*Returns the mean average precision (MAP) of all the queries*/
  lazy val meanAvePrec: Double

  /*Returns the normalized discounted cumulative gain for each query */
  lazy val ndcgAtK: RDD[Array[Double]]

  /**
   * param k the position to compute the truncated ndcg
   * return the average ndcg at the first k ranking positions
   */
  def ndcg(k: Int): Double
}
```

Author: coderxiang <shuoxiangpub@gmail.com>

Closes #2667 from coderxiang/rankingmetrics and squashes the following commits:

d881097 [coderxiang] update doc
14d9cd9 [coderxiang] remove unexpected files
d7fb93f [coderxiang] style change and remove ignored files
f113ee1 [coderxiang] modify doc for displaying superscript and subscript
f626896 [coderxiang] improve doc and remove unnecessary computation while labSet is empty
be6645e [coderxiang] set the precision of empty labset to 0.0
d64c120 [coderxiang] add logWarning for empty ground truth set
dfae292 [coderxiang] handle empty labSet for map. add test
62047c4 [coderxiang] style change and add documentation
f66612d [coderxiang] add additional test of precisionAt
b794cb2 [coderxiang] move private members precAtK, ndcgAtK into public methods. style change
77c9e5d [coderxiang] set precAtK and ndcgAtK as private member. Improve documentation
5f87bce [coderxiang] add API to calculate precision and ndcg at each ranking position
b7851cc [coderxiang] Use generic type to represent IDs
e443fee [coderxiang] change style and use alternative builtin methods
3a5a6ff [coderxiang] add ranking metrics

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