File headers: use SPDX license identifiers

add_curve_extra_objects/add_curve_braid.py

add_curve_extra_objects/add_curve_celtic_links.py

add_curve_extra_objects/add_curve_curly.py

add_curve_extra_objects/add_curve_spirals.py

add_curve_sapling/presets/callistemon.py

add_curve_sapling/presets/douglas_fir.py

add_curve_sapling/presets/japanese_maple.py

add_curve_sapling/presets/quaking_aspen.py

add_curve_sapling/presets/small_maple.py

add_curve_sapling/presets/small_pine.py

add_curve_sapling/presets/weeping_willow.py

add_curve_sapling/presets/white_birch.py

add_curve_sapling/presets/willow.py

add_mesh_extra_objects/Blocks.py

add_mesh_extra_objects/add_empty_as_parent.py

add_mesh_extra_objects/add_mesh_3d_function_surface.py

add_mesh_extra_objects/add_mesh_beam_builder.py

add_mesh_extra_objects/add_mesh_gears.py

add_mesh_extra_objects/add_mesh_gemstones.py

add_mesh_extra_objects/add_mesh_honeycomb.py