ReportUnit: fixed several Checkstyle issues (mvn checkstyle:checkstyle)

ReportUnit:
- Fixed several Checkstyle issues (after running "mvn checkstyle:checkstyle" and checking "target\checkstyle-result.xml").
- Re-enabled the new getComparator method as getComparatorV2.
- Fixed a bug for sorting on report index: the rest of the code is in an else branch now.
- Added a toString method for debugging purposes.

ReportUnitTest:
- Added the testComparator unit test and the supporting createReport method.

ViewerFrame:
- Added TODO: rename to SORT_KEY_COMPARE (since we use order for ascending/descending) and move to Constants.
- Changed else if into else in the sortChartFrameList method since the test was always true.

Constants:
- Removed two @SuppressWarnings("UnusedDeclaration") annotations because the lists are used in the unit tests.