Add significance model generator cli

[MariusArhaug]

I confirm that this contribution is made under the terms of the license found in the root directory of this repository's source tree and that I have the authority necessary to make this contribution on behalf of its copyright owner.

[bjorncs]

[ERROR] Tests run: 3, Failures: 2, Errors: 0, Skipped: 0, Time elapsed: 1.506 s <<< FAILURE! -- in com.yahoo.vespasignificance.SignificanceModelGeneratorTest
[ERROR] com.yahoo.vespasignificance.SignificanceModelGeneratorTest.testOverwriteExistingDocumentFrequencyLanguage -- Time elapsed: 0.069 s <<< FAILURE!
org.opentest4j.AssertionFailedError: expected: <1> but was: <2>
	at org.junit.jupiter.api.AssertionFailureBuilder.build(AssertionFailureBuilder.java:151)
	at org.junit.jupiter.api.AssertionFailureBuilder.buildAndThrow(AssertionFailureBuilder.java:132)
	at org.junit.jupiter.api.AssertEquals.failNotEqual(AssertEquals.java:197)
	at org.junit.jupiter.api.AssertEquals.assertEquals(AssertEquals.java:150)
	at org.junit.jupiter.api.AssertEquals.assertEquals(AssertEquals.java:145)
	at org.junit.jupiter.api.Assertions.assertEquals(Assertions.java:531)
	at com.yahoo.vespasignificance.SignificanceModelGeneratorTest.testOverwriteExistingDocumentFrequencyLanguage(SignificanceModelGeneratorTest.java:119)
	at java.base/java.lang.reflect.Method.invoke(Method.java:568)
	at java.base/java.util.ArrayList.forEach(ArrayList.java:1511)
	at java.base/java.util.ArrayList.forEach(ArrayList.java:1511)

[ERROR] com.yahoo.vespasignificance.SignificanceModelGeneratorTest.testGenerateSimpleFile -- Time elapsed: 0.014 s <<< FAILURE!
org.opentest4j.AssertionFailedError: expected: <1> but was: <2>
	at org.junit.jupiter.api.AssertionFailureBuilder.build(AssertionFailureBuilder.java:151)
	at org.junit.jupiter.api.AssertionFailureBuilder.buildAndThrow(AssertionFailureBuilder.java:132)
	at org.junit.jupiter.api.AssertEquals.failNotEqual(AssertEquals.java:197)
	at org.junit.jupiter.api.AssertEquals.assertEquals(AssertEquals.java:150)
	at org.junit.jupiter.api.AssertEquals.assertEquals(AssertEquals.java:145)
	at org.junit.jupiter.api.Assertions.assertEquals(Assertions.java:531)
	at com.yahoo.vespasignificance.SignificanceModelGeneratorTest.testGenerateSimpleFile(SignificanceModelGeneratorTest.java:60)
	at java.base/java.lang.reflect.Method.invoke(Method.java:568)
	at java.base/java.util.ArrayList.forEach(ArrayList.java:1511)
	at java.base/java.util.ArrayList.forEach(ArrayList.java:1511)

[MariusArhaug]

Resolved testing error by using junit temp directory annotation