Move stitching plugins to their own package
VisualizeTiles: extract Pt helper class

This class will be useful beyond just this single plugin;
better to have it in a toplevel class instead.
Pt: support double-precision width and height
Rename VisualizeTiles to TilesToROIs

The plugin changed focus, and is now less about visualization
and more about creating ROIs.

The next commit will add a new VisualizeTiles using VisAD
which is more in line with the goals of that plugin.
Add plugin for visualizing multi-tile data in 3D

This is a first cut which does not color the tiles properly,
but is otherwise ostensibly working.