A named list of userMatrix-style matrices used by
banc_view() / banc_front_view() / banc_vnc_view() etc. and
by banc_neuron_comparison_plot() to switch
nat.ggplot::geom_neuron() between standard BANC viewpoints
(main, side, front, vnc, vnc_side, brain_side). Exposed
so users can compose their own multi-panel ggplots without
re-deriving the matrices.