[SPARK-9914] [ML] define setters explicitly for Java and use setParam group in RFormula

The problem with defining setters in the base class is that it doesn't return the correct type in Java.

ericl

Author: Xiangrui Meng <meng@databricks.com>

Closes #8143 from mengxr/SPARK-9914 and squashes the following commits:

d36c887 [Xiangrui Meng] remove setters from model
a49021b [Xiangrui Meng] define setters explicitly for Java and use setParam group