ReportUnit:
- Fixed Checkstyle issues (after running "mvn checkstyle:checkstyle" and checking "target\checkstyle-result.xml").
- Major refactoring of the getComparator method.
- Removed unused methods: getReportErrorString and setReportErrorString.
- Commented out unused debugging method: printReportValues.

Constants:
- Added TODO: can we add SORT_KEY_MAX_INTENSITY [to the list LIST_SORT_KEYS_DOUBLE] as well {reportUnit.getChartUnit().getMaxTicIntensity()}?
- Created the LIST_SORT_KEYS_DOUBLE and LIST_SORT_KEYS_INT constants for combining sorting functionality for sort keys with the same type.