[SPARK-9493] [ML] add featureIndex to handle vector features in IsotonicRegression

This PR contains the following changes:
* add `featureIndex` to handle vector features (in order to chain isotonic regression easily with output from logistic regression
* make getter/setter names consistent with params
* remove inheritance from Regressor because it is tricky to handle both `DoubleType` and `VectorType`
* simplify test data generation

jkbradley zapletal-martin

Author: Xiangrui Meng <meng@databricks.com>

Closes #7952 from mengxr/SPARK-9493 and squashes the following commits:

8818ac3 [Xiangrui Meng] address comments
05e2216 [Xiangrui Meng] address comments
8d08090 [Xiangrui Meng] add featureIndex to handle vector features make getter/setter names consistent with params remove inheritance from Regressor