Finalize SDFormat 1.10 specification

Refer to: [SDFormat 1.10](http://sdformat.org/spec?elem=sdf&ver=1.10).

Adding new elements is fairly easy, we do accept pull requests.

### sdf

- [ ] `sdf/actor`
- [ ] `sdf/light`

### world

- [ ] `world/audio`
- [ ] `world/wind`
- [ ] `world/include`
- [ ] `world/atmosphere`
- [ ] `world/gui`
- [ ] `world/light`
- [x] `world/frame` https://github.com/ami-iit/rod/pull/12
- [ ] `world/actor`
- [ ] `world/plugin`
- [ ] `world/road`
- [ ] `world/spherical_coordinates`
- [ ] `world/state`
- [ ] `world/population`

### scene

- [ ] `scene/sky`
- [ ] `scene/fog`

### state

- [ ] `state`

### physics

- [ ] `physics/dart`
- [ ] `physics/simbody`
- [ ] `physics/bullet`
- [ ] `physics/ode`

### light

- [ ] `light`

### actor

- [ ] `actor`

### model

- [x] `model/frame` https://github.com/ami-iit/rod/pull/12
- [ ] `model/plugin`
- [ ] `model/gripper`

### link

- [ ] `link/velocity_decay`
- [ ] `link/sensor`
- [ ] `link/projector`
- [ ] `link/audio_sink`
- [ ] `link/audio_source`
- [ ] `link/battery`
- [ ] `link/light`
- [ ] `link/particle_emitter`

### sensor

- [ ] `sensor`

### joint

- [ ] `joint/gearbox_ratio`
- [ ] `joint/gearbox_reference_body`
- [ ] `joint/thread_pitch`
- [ ] `joint/axis2`
- [ ] `joint/physics`
- [ ] `joint/sensor`

### collision

- [ ] `collision/laser_retro`
- [ ] `collision/max_contacts`
- [ ] `collision/surface`

### visual

- [ ] `visual/cast_shadows`
- [ ] `visual/laser_retro`
- [ ] `visual/transparency`
- [ ] `visual/visibility_flags`
- [ ] `visual/meta`
- [ ] `visual/plugin`

### material

- [ ] `material/shader`
- [ ] `material/render_order`
- [ ] `material/double_sided`
- [ ] `material/pbr`

### geometry

- [ ] `geometry/empty`
- [ ] `geometry/heightmap/texture`
- [ ] `geometry/heightmap/blend`
- [ ] `geometry/heightmap/use_terrain_paging`
- [ ] `geometry/heightmap/sampling`
- [ ] `geometry/image`
- [ ] `geometry/mesh/submesh`
- [ ] `geometry/polyline`