arslan is now a ggseg_atlas object (from ggseg.formats) containing
both 2D and 3D data. The separate arslan_3d object has been removed.
Use ggplot() + ggseg::geom_brain(atlas = arslan) for 2D plots and
ggseg3d::ggseg3d(atlas = arslan) for 3D plots — both from the same
object.
ggseg.formats is now a hard dependency (in Depends).
Package URLs updated from LCBC-UiO to ggseg GitHub organisation.