sonar clean up suggestions

src/main/java/com/xeiam/xchart/AxisTick.java

src/main/java/com/xeiam/xchart/ChartLegend.java

src/main/java/com/xeiam/xchart/interfaces/IChartPart.java

src/main/java/com/xeiam/xchart/interfaces/IHideable.java

src/main/java/com/xeiam/xchart/series/Series.java