Modify greybox concept to 3D modeling

Rename the greybox concept to 3D modeling (0.9.0) (#52)

The panel scene is built by the 3D modeling stage now, so the code, docs and
tests say so: panel_greybox -> panel_modeling, greybox_gate -> modeling_gate,
make_panel_greybox -> make_panel_modeling, Paths.greyboxes_dir -> modeling_dir,
PanelGreyboxSpec -> PanelModelingSpec, and the Blender kernel 'greybox' ->
'modeling'. Prose says "3D model". The on-disk layout is unchanged.