Create extensible SimObject that is capable of loading meshes

[zhiquanyeo]

As part of a longer term refactor, look into making a SimObject subclass that is capable of loading a custom mesh from a glTF file. Once this is done, we can port SimRobot to extend from this instead of the base SimObject.

This might entail reworking the way we add meshes to the scene as mesh loading is inherently async

[JamesWP]

Take a look at the solution on the object loading example in the FRUK-Simulator GitHub org.

It shows a method of creating an empty parent mesh and async adding the robot to it when it loads

[zhiquanyeo]

Yeah that's the way I have it set up in SimRobot for custom meshes. We will need to handle async loads if we try to interpret the geometry of physics bodies from a custom mesh though