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

[mengxr]

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.
2. Move shared params to package ml.param.shared.
3. Set default values in Params instead of in Param.
4. Add a few methods to Params and ParamMap.
5. Move schema handling to SchemaUtils from Params.

• check visibility of the methods added

[SparkQA]

Test build #29906 has finished for PR 5431 at commit 29b004c.

• This patch fails to build.
• This patch does not merge cleanly.
• This patch adds no public classes.
• This patch does not change any dependencies.

[SparkQA]

Test build #29910 has finished for PR 5431 at commit 55be1f3.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.
• This patch does not change any dependencies.

[SparkQA]

Test build #29909 has finished for PR 5431 at commit d63b5cc.

• This patch passes all tests.
• This patch does not merge cleanly.
• This patch adds no public classes.
• This patch does not change any dependencies.

[SparkQA]

Test build #30150 has finished for PR 5431 at commit eec2264.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.
• This patch does not change any dependencies.

[jkbradley]

Should this method in Params be made abstract?

def validate(paramMap: ParamMap): Unit = {}

I just realized we haven't been using it, and making it abstract would force users to think about it.

[jkbradley]

Add test for "params.clear"

[mengxr]

done

[jkbradley]

"Make sure that the input param is initialized before this gets called." will be unclear to new developers. Maybe move this to parameter-specific doc:

@param param:  Make sure that this param is initialized before this method gets called.

[mengxr]

done

[jkbradley]

@mengxr I added minor comment but don't have major ones. That's it for now!

[jkbradley]

LGTM once tests pass

[SparkQA]

Test build #30210 has finished for PR 5431 at commit 26ae2d7.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.
• This patch does not change any dependencies.

[SparkQA]

Test build #30212 has finished for PR 5431 at commit d19236d.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.
• This patch does not change any dependencies.

[mengxr]

Merged into master.