[SPARK-17188][SQL] Moves class QuantileSummaries to project catalyst for implementing percentile_approx

[clockfly]

What changes were proposed in this pull request?

This is a sub-task of SPARK-16283 (Implement percentile_approx SQL function), which moves class QuantileSummaries to project catalyst so that it can be reused when implementing aggregation function percentile_approx.

How was this patch tested?

This PR only does class relocation, class implementation is not changed.

[SparkQA]

Test build #64217 has finished for PR 14754 at commit 8ae3789.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • class QuantileSummaries(
  • case class Stats(value: Double, g: Int, delta: Int)

[rxin]

Are there tests that we'd need to move?

[clockfly]

@rxin updated.

[SparkQA]

Test build #64268 has finished for PR 14754 at commit e8745e7.

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

[cloud-fan]

thanks, merging to master!