[FEATURE] Add implicit fem with newton + conjugate gradient

[Libero0809]

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Description

Added an implicit Euler integration method with Newton + Conjugate Gradient Solver. Also added a test for this solver.

Related Issue

Resolves Genesis-Embodied-AI/Genesis#1182

Motivation and Context

Implicit solver can offer a more robust and stable simulation with a larger time step.

How Has This Been / Can This Be Tested?

I tested on my MacBook. The implicit method can use real dt = 1e-2 to simulate while explicit method needs real dt=1e-4 (dt = 1e-3, substeps=10) to run.

import genesis as gs
import time
gs.init(backend=gs.cpu, precision="32", logging_level="debug")

scene = gs.Scene(
    sim_options=gs.options.SimOptions(
        dt=0.01,
        substeps=1,
    ),
    fem_options=gs.options.FEMOptions(
        use_explicit_solver=True,
        n_newton_iterations=3,
        n_pcg_iterations=50,
        pcg_threshold=1e-4,
    ),
    show_viewer=True,
)

scene.add_entity(
    material=gs.materials.FEM.Elastic(),
    morph=gs.morphs.Mesh(
        file="./cube8.obj",
        scale=0.1,
        pos=(0, 0, 0.5),
    ),
)
scene.build()

for i in range(500):
    scene.step()

cube8.zip @YilingQiao