Support Mouseover on Points

• implement screen-space BSP of scatter points
• implement greedy search of BSP given a mouse coordinate
• implement complete search of BSP given a mouse corodinate
• Plot provide computed ref of single nearest plot point to mouse in threshold distance
  • add ability to provide sorted list of nearby points, instead of greedily returning the first found inside the threshold (this will allow finer grained plot values to be hit)
• visually indicate nearby points