Eliminate tracking of binary ImagePluses

We now always make 8-bit displays from 8-bit ImagePluses even if they are
binary (0/255) structures. This means that in order to get 1-bit the user
must explicitly run the type change plugin to 1-bit.

1-bit dataset data always maps to 8-bit binary ImagePluses and that mapping
is maintained via clamping (0,1) and promotion (0,255) by the pixel
harmonizers as needed.

These changes simplify the harmonization code and also can greatly improve
performance for degenerate cases with virtual data structures.