Add support for violin plot

[LUUUAN]

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Description

This is the fix for issue #509

Two files are added: ViolinPlot.java and ViolinTrace.java. Also I modified the file trace_template.html so that violin plot can draw the inner box plot at the same time. Also, considering that the mean line of violin plot is also frequently used, it is also added into the trace_template.html but I comment the code (context.put("meanLine", "{visible: true}")) that draw the mean line in ViolinTrace.java because it doesn't perform well together with inner box plot. You can changed the code and style according to your preference.

Testing

Test is developed following the style of other plot tests.

[lwhite1]

the box and meanline options should be added to the builder rather than always including one and excluding the other

[LUUUAN]

Ok

[LUUUAN]

I have added two method to ViolinTrace.java to configure whether box plot and mean line will show up in the figure.

[lwhite1]

@LUUUAN This looks good. Thank you