Dynamic particle injection and dynamic meshes

[shi-eric]

Discussed in https://github.com/NVIDIA/warp/discussions/207

^{Originally posted by anthropoy April 10, 2024} By using the model builder to build a sim, the particle numbers and meshes have to be finalised and considered to be invariant throughout the sim. Can the model class be expanded to include particle injection (and deletion), and also add dynamic properties to meshes, e.g. translations and rotations. Thanks!