Add backlash model for gear mesh

[jguarato]

Motivation

Gear pairs with clearance introduce discontinuous contact forces that a linear mesh stiffness cannot capture. This PR enables backlash-aware multi-rotor simulations while keeping the previous linear mesh coupling when backlash is disabled.

Changes

Multi-rotor package

• Reorganize multi-rotor code into ross/multi_rotor/ (MultiRotor, GearElement, Mesh)
• Introduce Backlash (force calculation for Newmark) in Mesh.
• Add an optional gear backlash model for time-response analysis (rigid or smooth approach), including transmission error and dynamic mesh forces injected into Newmark integration.
• MultiRotor accepts a backlash dict (enable, initial, error_amp, smooth_operator, sigma).
• When backlash is enabled, mesh stiffness is applied via nonlinear forces instead of the linear coupling matrix.

Improve transient integration

• More robust Newmark path for stiff/nonlinear systems
• Support newmark_type="simple"|"robust", and fix coupling/force assembly so backlash works correctly in MultiRotor.

Usage sketch

multi_rotor = rs.MultiRotor(
    rotor1,
    rotor2,
    coupled_nodes=(4, 0),
    gear_mesh_stiffness=1e8,
    backlash={
        "enable": True,
        "initial_value": 1e-4,
        "error_amp": 0.0,
        "smooth_operator": False,
        "sigma": 1e4,
    },
)
results = multi_rotor.run_time_response(..., method="newmark", newmark_type="robust")