Added FormattedDouble Class

[capak07]

Purpose

Description

• Fixes #1769
• Added FormattedDouble Class that'll help write double numbers using gson

Checklist

• [ ] New code follows the Google Java Style Guide\ This is automatically checked by mvn verify, but can also be checked on its own using mvn spotless:check.\ Style violations can be fixed using mvn spotless:apply; this can be done in a separate commit to verify that it did not cause undesired changes.
• [ ] If necessary, new public API validates arguments, for example rejects null
• [ ] New public API has Javadoc
  • [ ] Javadoc uses @since $next-version$
    ($next-version$ is a special placeholder which is automatically replaced during release)
• [ ] If necessary, new unit tests have been added
  • [ ] Assertions in unit tests use Truth, see existing tests
  • [ ] No JUnit 3 features are used (such as extending class TestCase)
  • [ ] If this pull request fixes a bug, a new test was added for a situation which failed previously and is now fixed
• [ ] mvn clean verify javadoc:jar passes without errors

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[eamonnmcmanus]

This is just copied from a comment in #1769. It's not connected to anything, there are no tests, and it does not build because of -source 7.