[SPARK-5019] Update GMM API to use MultivariateGaussian

[Lewuathe]

GMM should have the public accessor for MultivariateGaussian model list.
With this api, gaussian parameters can be obtain through MultivariateGaussian. Therefore previous mu and sigma array become private.

[SparkQA]

Test build #25103 has started for PR 3911 at commit 930a98e.

• This patch merges cleanly.

[SparkQA]

Test build #25103 has finished for PR 3911 at commit 930a98e.

• This patch fails Scala style tests.
• This patch merges cleanly.
• This patch adds no public classes.

[AmplabJenkins]

Test FAILed.
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Test FAILed.

[SparkQA]

Test build #25104 has started for PR 3911 at commit 6b177dc.

• This patch merges cleanly.

[SparkQA]

Test build #25104 has finished for PR 3911 at commit 6b177dc.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[AmplabJenkins]

Test PASSed.
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Test PASSed.

[jkbradley]

@Lewuathe Could you please close this PR since it was fixed by [https://github.com//pull/4088]? Thanks!

[Lewuathe]

@jkbradley Sorry. I close this PR. Thank you.