PBD3DEntity（PBD 3D 实体）

class genesis.engine.entities.pbd_entity.PBD3DEntity(scene, solver, material, morph, surface, particle_size, idx, particle_start, edge_start, elem_start, vvert_start, vface_start)

基类：PBDTetEntity

3D mesh entity.

sample()

property n_elems

property elem_start

property elem_end