view mesh and skeleton together in 3d with microviewer

[huangziwei]

Added optional support for microviewer (pip install "skeliner[3d]").

import skeliner as sk
sk.plot.view3d(skel, mesh, box=None) 
              # self-defined bounding box, default is None, 
              # which will use the extends of the mesh
# alternatively sk.plot3d(skel, mesh, box=(0,0,0, 1200, 1200, 100))

But we need to wait for an upstream fix of the mesh opacity before merging.

seung-lab/microviewer#11

(turns out, this is not a bug in microviewer but we need to provide an id to keep track of the pair. fixed in #8bd0bf0)

we can still wait until the next microviewer release (which will add right click camera recentering) but this can definitely be merged now.