[mllib] DecisionTree Strategy parameter checks

Added some checks to Strategy to print out meaningful error messages when given invalid DecisionTree parameters.
CC mengxr

Author: Joseph K. Bradley <joseph.kurata.bradley@gmail.com>

Closes #1821 from jkbradley/dt-robustness and squashes the following commits:

4dc449a [Joseph K. Bradley] Merge remote-tracking branch 'upstream/master' into dt-robustness
7a61f7b [Joseph K. Bradley] Added some checks to Strategy to print out meaningful error messages when given invalid DecisionTree parameters
[SPARK-2852][MLLIB] Separate model from IDF/StandardScaler algorithms

This is part of SPARK-2828:

1. separate IDF model from IDF algorithm (which generates a model)
2. separate StandardScaler model from StandardScaler

CC: dbtsai

Author: Xiangrui Meng <meng@databricks.com>

Closes #1814 from mengxr/feature-api-update and squashes the following commits:

40d863b [Xiangrui Meng] move mean and variance to model
48a0fff [Xiangrui Meng] separate Model from StandardScaler algorithm
89f3486 [Xiangrui Meng] update IDF to separate Model from Algorithm