fix bug in plot3d.hclust when single colour specified

* include test
ensure that nat::plot3d.character is called

* this is a hack which should be removed in due course but is required because
  plot3d.character is presently aliased to plot3dfc in some circumstances
Depend on nat 1.5.7 to ensure plot3d.hclust(,soma=T) works
Bump version to 1.2