Add function to screen neurons in 3D

So far this is just a straightforward port of ``dpscan()`` from [flycircuit](https://github.com/jefferis/flycircuit) with the ``plot3dfc()`` call replaced with the ``plot3d()`` equivalent. R's lazy-evaluation made this less trivial than expected, but the difference is just:
```r
pl <- do.call(plot3d, args=list(x=n, col=ifelse(n %in% selected, selected_col, col[i]), flip=flip[i], ...))
```
rather than:
```r
pl <- plot3dfc(n, col=ifelse(n %in% selected, selected_col, col[i]), flip=flip[i], ...)
```