* OgreSlice has a slice_id.
This is necessary because we will have multiple OgreSlices
and Ogre requires unique names for textures and other entities.
* Visualize both high and low res slices.
Adds checkboxes to the rviz plugin to show high and low
resolution slices.
DrawableSubmap now holds a vector of OgreSlices and
decides which slices are visible.
* Corrections. Move slice visibility to OgreSlice.
Renames OgreSubmap to OgreSlice in preparation for that a
DrawableSubmap should hold multiple OgreSlices, so multiple
types of slices for one submap can be visualized.