Add curved step line edges

[mariogrip]

This adds curved step line edges, putting this as a draft to get some feedback.

[imaNNeo]

Hi, thanks for your contribution. I'm sorry about my late reply.
It looks good to me,
I think we can remove isStepLineEdgesCurved property, as we can having it using our stepLineEdgesSmoothness. It can be stepLineEdgesSmoothness != 0

I think you need to rebase your branch.

Please update the bar_chart.md file to add this property,
Also don't forget to write a unit-test for your feature