vol*: 3D slicer

[alisterburt]

This was identified as a high value volume related feature in #1086

A 2D viewer capable of viewing arbitrary slices through the 3D data is a generally useful component

Here is a good description of the one in IMOD from bio3d.colorado.edu...

The slicer allows you to display a slice at an arbitrary angle through the volume, centered around a selected point. This center of rotation is always displayed in the center of the window. Multiple slices can be averaged. Modeling can be done.