add more visualization options for BoundingBox and BoundingBoxArray

[joachim-p]

• alpha = value mode
• rescale alpha to [alpha_min alpha_max] range
• remove all boxes with value < threshold
• fix unintended shape_ state behaviour in case box size constraints are violated

[k-okada]

@joachim-p thanks for contribution, could you update document too ? https://github.com/jsk-ros-pkg/jsk_visualization/blob/master/doc/jsk_rviz_plugins/plugins/bounding_box_array.rst