First attempt at virtual stack support

This code uses Imglib's virtual stack implementations. Img's derived from
AbstractCellImg go to IJ2 as virtual stacks.

Note that there is not immediately straightforward implementation for our
support of COLOR_RGB images. IJ2 relies on all data being RealType derived.
ARGBType is not and cannot be the basis of a Dataset. Instead we use support
3-channel byte images that translate as COLOR_RGB in the legacy layer. To
allow virtual stacks of this kind of data we'll need to come up with a special
virtual stack impl.