Staggered flashing

[hubbardp]

Stephan Saalfeld (https://github.com/axtimwalde) suggested that there should be a version of "fade" with "stagger" that would not just show all the meshes with staggered timing, or just hide them with staggered timing, but "flash" them from hidden to shown to hidden again with staggered timing. The effect would be similar to the "flash" command for the version of neuVid that works with VVD Viewer (https://github.com/connectome-neuprint/neuVid/blob/master/documentation/README_VVD.md#flash) but with a single command replacing a series of commands to operate on a set of meshes.