ChartUnit: fixed Checkstyle issues (mvn checkstyle:checkstyle)

- Fixed Checkstyle issues (after running "mvn checkstyle:checkstyle" and checking "target\checkstyle-result.xml").
- Changed MAX_INTENSITY_STRING_FORMAT and RANGE_AXIS_NUMBER_FORMAT into MAX_INTENSITY_FORMAT and RANGE_AXIS_FORMAT to allow higher level future reuse.