MultiPoint interactivity/spatial indexing

[luucvanderzee]

I could do this quickly right now, but it is probably better to take this on when doing #80. Like with many types of lines, for the multipoint you would want to split up the mark and index it in parts, instead of the current method where you index its bounding box and then do a lot of collision detection. This whole idea of splitting up marks and indexing them separately is just not possible without a good refactor