Profile picking in instrument view / 3d view

[nvaytet]

See this example in pythreejs: https://github.com/jupyter-widgets/pythreejs/blob/master/examples/Picker.ipynb

Unfortunately, the index of the point over which the cursor is hovering is not returned by pythreejs (a point cloud has no faces), only the coordinate of the cursor is returned.
Performing a where search on all the pixels to find the smallest distance would be too slow, so we would have to use sc.bin to overlay a 3d grid onto the pixel positions, to then find the mininum distance on only the pixels inside the bin.

[nvaytet]

Seems there was progress on the pythreejs side in the mean time that could make this easier to implement: jupyter-widgets/pythreejs#379