USD scatter export doesn't support error bars

[Carifio24]

Currently the USD scatter export doesn't support error bars. For the GL exports we use the LINES rendering mode (which unsurprisingly matches the vispy lines quite nicely). AFAICT there isn't a corresponding primitive type for USD. Currently we just ignore error bars in this case, but we should come up with some way to support these. We have functionality for defining cylinder meshes, so one possible path is to just use very thin (defined in some appropriate way) cylinders to create the error bars.