[SPARK-5957][ML] better handling of parameters

The design doc was posted on the JIRA page. Python changes will be in a follow-up PR. jkbradley

1. Use codegen for shared params.
1. Move shared params to package `ml.param.shared`.
1. Set default values in `Params` instead of in `Param`.
1. Add a few methods to `Params` and `ParamMap`.
1. Move schema handling to `SchemaUtils` from `Params`.

- [x] check visibility of the methods added

Author: Xiangrui Meng <meng@databricks.com>

Closes #5431 from mengxr/SPARK-5957 and squashes the following commits:

d19236d [Xiangrui Meng] fix test
26ae2d7 [Xiangrui Meng] re-gen code and mark clear protected
38b78c7 [Xiangrui Meng] update Param.toString and remove Params.explain()
409e2d5 [Xiangrui Meng] address comments
2d637bd [Xiangrui Meng] Merge remote-tracking branch 'apache/master' into SPARK-5957
eec2264 [Xiangrui Meng] make get* public in Params
4090d95 [Xiangrui Meng] Merge remote-tracking branch 'apache/master' into SPARK-5957
4fee9e7 [Xiangrui Meng] re-gen shared params
2737c2d [Xiangrui Meng] rename SharedParamCodeGen to SharedParamsCodeGen
e938f81 [Xiangrui Meng] update code to set default parameter values
28ed322 [Xiangrui Meng] merge master
55be1f3 [Xiangrui Meng] merge master
d63b5cc [Xiangrui Meng] fix examples
29b004c [Xiangrui Meng] update ParamsSuite
94fd98e [Xiangrui Meng] fix explain params
48d0e84 [Xiangrui Meng] add remove and update explainParams
4ac6348 [Xiangrui Meng] move schema utils to SchemaUtils add a few methods to Params
0d9594e [Xiangrui Meng] add getOrElse to ParamMap
eeeffe8 [Xiangrui Meng] map ++ paramMap => extractValues
0d3fc5b [Xiangrui Meng] setDefault after param
a9dbf59 [Xiangrui Meng] minor updates
d9302b8 [Xiangrui Meng] generate default values
1c72579 [Xiangrui Meng] pass test compile
abb7a3b [Xiangrui Meng] update default values handling
dcab97a [Xiangrui Meng] add codegen for shared params