SPARK-4111 [MLlib] add regression metrics

Add RegressionMetrics.scala as regression metrics used for evaluation and corresponding test case RegressionMetricsSuite.scala.
safer to do the following (though `summarizer.normL2(1)` is not optimized):

~~~
rmse = summarizer.normL2(1) / math.sqrt(summarizer.count)
rmse * rmse
~~~