Add soft landing problem with conic constraints

[bjack205]

Doing a quadrotor instead, since it ends up being pretty much the same model.

Details Has an initial downward velocity of -3 m/s. Uses the rate-limited quadrotor model. Solved for state trajectories using COSMO.

Problem details:

• time horizon: 3 seconds
• nhorizon: 101
• nstates: 18
• ncontrols: 4
• ADMM size: 1818 / 404

Without / Without SOC constraint:

• iterations: 74 / 67
• solve time: 1.93 / 4.58 s