PBD2DEntity（PBD 2D 实体）

class genesis.engine.entities.pbd_entity.PBD2DEntity(scene, solver, material, morph, surface, particle_size, idx, particle_start, edge_start, inner_edge_start, vvert_start, vface_start)

基类：PBDTetEntity

2D mesh entity.

sample()

property n_inner_edges