allow slabinterval normalization to be for all, by panel, by group, or none

[mjskay]

Currently the values of the normalize argument are pretty opaque. Should probably be something like:

• normalize = "all" (currently "max_height"): normalize to max height over everything
• normalize = "panel" (no current analog): normalize to max height within-panel
• normalize = "group" (currently "height"): normalize to max height within-group
• normalize = "none" (currently "none"): no normalization