Add new physics elements for Nvidia PhysX

[hyunseok-yang]

Desired behavior

I hope SDFormat would support the PhysX(Nvidia) as a physic engine so that I can utilize the standard format for robot simulation.

Since Nvidia PhysX introduced a new articulation body for robotics, and I think it's enough and possible to present the robot s in the world what we imagine.

refer to example here.

TGS is provided for solver in PhysX.

I guess many developers already know these feature in PhysX 4.0 but current SDFormat(even latest version) does not support that type in official. :(

Anyone have the plan to introduce a new physics engine type?

Implementation suggestion

describe any parameters related to PhysX

Additional context

Actually I'm working on open project which called CLOiSim to simulate the robotics simulation using SDFormat.
I couldn't find reasonable SDFormat parsing library for C#, so I've created the parser by my self.

And it implements the physics things like joint, inertial and whatever.. using the parameters in ODE type.

[chapulina]

Adding specific elements for every physics engine hasn't been scaling well. I believe the direction we're planning to take is to use generic elements instead of engine-specific ones, see #508.