[SPARK-6787] [ML] add read/write to estimators under ml.feature (1)

[mengxr]

Add read/write support to the following estimators under spark.ml:

• CountVectorizer
• IDF
• MinMaxScaler
• StandardScaler (a little awkward because we store some params in spark.mllib model)
• StringIndexer

Added some necessary method for read/write. Maybe we should add private[ml] trait DefaultParamsReadable and DefaultParamsWritable to save some boilerplate code, though we still need to override load for Java compatibility.

@jkbradley

[mengxr]

Included some code clean-up of #6665.

[SparkQA]

Test build #46190 has finished for PR 9798 at commit 3ea051a.

• This patch fails Scala style tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):\n * class CountVectorizerModelWriter(instance: CountVectorizerModel) extends Writer\n * final class IDF(override val uid: String) extends Estimator[IDFModel] with IDFBase with Writable\n * class MinMaxScalerModelWriter(instance: MinMaxScalerModel) extends Writer\n * class StandardScalerModelWriter(instance: StandardScalerModel) extends Writer\n * class StringIndexModelWriter(instance: StringIndexerModel) extends Writer\n

[jkbradley]

They are already Params. This is just weird b/c the new and old model params can get out of synch. Fine for now though.

[yanboliang]

I send #9839 to do minor refactor for StandardScaler, it fixed this issue.

[jkbradley]

@mengxr LGTM except for the Scala style issue.

My comments aren't actionable unless you want to simplify the tests using testEstimatorAndModelReadWrite

[mengxr]

testEstimatorAndModelReadWrite calls fit, which takes some extra time. So I construct the model directly.

[mengxr]

test this please

[mengxr]

test this please

[SparkQA]

Test build #46262 has finished for PR 9798 at commit a41349f.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):\n * class CountVectorizerModelWriter(instance: CountVectorizerModel) extends Writer\n * final class IDF(override val uid: String) extends Estimator[IDFModel] with IDFBase with Writable\n * class MinMaxScalerModelWriter(instance: MinMaxScalerModel) extends Writer\n * class StandardScalerModelWriter(instance: StandardScalerModel) extends Writer\n * class StringIndexModelWriter(instance: StringIndexerModel) extends Writer\n

[mengxr]

Merged into master and branch-1.6.