[Feature] add particle free in MPM

[zswang666]

Feature support for #190

New APIs: mpm_entity.set/get_free.
Non-free particles behave as boundary conditions.

Example code

import genesis as gs

########################## init ##########################
gs.init()

########################## create a scene ##########################

scene = gs.Scene(
    sim_options=gs.options.SimOptions(
        dt=4e-3,
        substeps=10,
    ),
    mpm_options=gs.options.MPMOptions(
        lower_bound=(-0.5, -1.0, 0.0),
        upper_bound=(0.5, 1.0, 1),
    ),
    vis_options=gs.options.VisOptions(
        visualize_mpm_boundary=True,
    ),
    viewer_options=gs.options.ViewerOptions(
        camera_fov=30,
        res=(960, 640),
    ),
    show_viewer=True,
)

########################## entities ##########################
plane = scene.add_entity(
    morph=gs.morphs.Plane(),
)

obj = scene.add_entity(
    material=gs.materials.MPM.Elastic(),
    morph=gs.morphs.Box(
        pos=(0.0, -0.5, 0.25),
        size=(0.2, 0.5, 0.2),
    ),
    surface=gs.surfaces.Default(
        color=(1.0, 0.4, 0.4),
        vis_mode="particle",
    ),
)

########################## build ##########################
scene.build()

################### set active/free/non-fixed particles ####################
pos = obj.get_state().pos.detach().cpu()
is_fixed = pos[:, 1] < (pos.min(0)[0][1] + 0.2)
free = obj.get_free()
free[is_fixed] = 0
obj.set_free(free)

########################## run ##########################
for i in range(1000):
    scene.step()