[SPARK-6113] [ml] Stabilize DecisionTree API

This is a PR for cleaning up and finalizing the DecisionTree API.  PRs for ensembles will follow once this is merged.

### Goal

Here is the description copied from the JIRA (for both trees and ensembles):

> **Issue**: The APIs for DecisionTree and ensembles (RandomForests and GradientBoostedTrees) have been experimental for a long time. The API has become very convoluted because trees and ensembles have many, many variants, some of which we have added incrementally without a long-term design.
> **Proposal**: This JIRA is for discussing changes required to finalize the APIs. After we discuss, I will make a PR to update the APIs and make them non-Experimental. This will require making many breaking changes; see the design doc for details.
> **[Design doc](https://docs.google.com/document/d/1rJ_DZinyDG3PkYkAKSsQlY0QgCeefn4hUv7GsPkzBP4)** : This outlines current issues and the proposed API.

Overall code layout:
* The old API in mllib.tree.* will remain the same.
* The new API will reside in ml.classification.* and ml.regression.*

### Summary of changes

Old API
* Exactly the same, except I made 1 method in Loss private (but that is not a breaking change since that method was introduced after the Spark 1.3 release).

New APIs
* Under Pipeline API
* The new API preserves functionality, except:
  * New API does NOT store prob (probability of label in classification).  I want to have it store the full vector of probabilities but feel that should be in a later PR.
* Use abstractions for parameters, estimators, and models to avoid code duplication
* Limit parameters to relevant algorithms
* For enum-like types, only expose Strings
  * We can make these pluggable later on by adding new parameters.  That is a far-future item.

Test suites
* I organized DecisionTreeSuite, but I made absolutely no changes to the tests themselves.
* The test suites for the new API only test (a) similarity with the results of the old API and (b) elements of the new API.
  * After code is moved to this new API, we should move the tests from the old suites which test the internals.

### Details

#### Changed names

Parameters
* useNodeIdCache -> cacheNodeIds

#### Other changes

* Split: Changed categories to set instead of list

#### Non-decision tree changes
* AttributeGroup
  * Added parentheses to toMetadata, toStructField methods (These were removed in a previous PR, but I ran into 1 issue with the Scala compiler not being able to disambiguate between a toMetadata method with no parentheses and a toMetadata method which takes 1 argument.)
* Attributes
  * Renamed: toMetadata -> toMetadataImpl
  * Added toMetadata methods which return ML metadata (keyed with “ML_ATTR”)
  * NominalAttribute: Added getNumValues method which examines both numValues and values.
* Params.inheritValues: Checks whether the parent param really belongs to the child (to allow Estimator-Model pairs with different sets of parameters)

### Questions for reviewers

* Is "DecisionTreeClassificationModel" too long a name?
* Is this OK in the docs?
```
class DecisionTreeRegressor extends TreeRegressor[DecisionTreeRegressionModel] with DecisionTreeParams[DecisionTreeRegressor] with TreeRegressorParams[DecisionTreeRegressor]
```

### Future

We should open up the abstractions at some point.  E.g., it would be useful to be able to set tree-related parameters in 1 place and then pass those to multiple tree-based algorithms.

Follow-up JIRAs will be (in this order):
* Tree ensembles
* Deprecate old tree code
* Move DecisionTree implementation code to new API.
* Move tests from the old suites which test the internals.
* Update programming guide
* Python API
* Change RandomForest* to always use bootstrapping, even when numTrees = 1
* Provide the probability of the predicted label for classification.  After we move code to the new API and update it to maintain probabilities for all labels, then we can add the probabilities to the new API.

CC: mengxr  manishamde  codedeft  chouqin  MechCoder

Author: Joseph K. Bradley <joseph@databricks.com>

Closes #5530 from jkbradley/dt-api-dt and squashes the following commits:

6aae255 [Joseph K. Bradley] Changed tree abstractions not to take type parameters, and for setters to return this.type instead
ec17947 [Joseph K. Bradley] Updates based on code review.  Main changes were: moving public types from ml.impl.tree to ml.tree, modifying CategoricalSplit to take an Array of categories but store a Set internally, making more types sealed or final
5626c81 [Joseph K. Bradley] style fixes
f8fbd24 [Joseph K. Bradley] imported reorg of DecisionTreeSuite from old PR.  small cleanups
7ef63ed [Joseph K. Bradley] Added DecisionTreeRegressor, test suites, and example (for real this time)
e11673f [Joseph K. Bradley] Added DecisionTreeRegressor, test suites, and example
119f407 [Joseph K. Bradley] added DecisionTreeClassifier example
0bdc486 [Joseph K. Bradley] fixed issues after param PR was merged
f9fbb60 [Joseph K. Bradley] Done with DecisionTreeClassifier, but no save/load yet.  Need to add example as well
2532c9a [Joseph K. Bradley] partial move to spark.ml API, not done yet
c72c1a0 [Joseph K. Bradley] Copied changes for common items, plus DecisionTreeClassifier from original PR

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