[SPARK-9005][MLlib]Fix RegressionMetrics computation of explainedVariance

[feynmanliang]

Fixes implementation of explainedVariance and r2 to be consistent with their definitions as described in SPARK-9005.

[SparkQA]

Test build #37104 has finished for PR 7361 at commit 4c4e56f.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #37106 has finished for PR 7361 at commit bde9761.

• This patch fails Scala style tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #37109 has finished for PR 7361 at commit db8605a.

• This patch fails PySpark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #37118 has finished for PR 7361 at commit 08a0e1b.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[srowen]

.sum?

[feynmanliang]

Thanks!

[SparkQA]

Test build #37140 has finished for PR 7361 at commit 1a3d098.

• This patch fails Python style tests.
• This patch merges cleanly.
• This patch adds no public classes.

[feynmanliang]

jenkins retest this please

[SparkQA]

Test build #37149 has finished for PR 7361 at commit 1a3d098.

• This patch fails Python style tests.
• This patch merges cleanly.
• This patch adds no public classes.

[jkbradley]

Could you put the formula here please?

[jkbradley]

LGTM pending tests (except for missing doc)

[SparkQA]

Test build #37174 has finished for PR 7361 at commit f1112fc.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[srowen]

LGTM

[SparkQA]

Test build #1070 has finished for PR 7361 at commit f1112fc.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[jkbradley]

@feynmanliang Could you please modify the title and description so they just say that explainedVariance is being fixed? (r2 is not changing, right?)

[jkbradley]

Merging with master
Thanks!