Shakes players' screens - and optionally physics objects - around a point. Half-Life's
env_shake. Call Sandbox.Mapping.EnvShake.StartShake to set it off on every player, and it runs a
Sandbox.Mapping.EnvShake.Duration second cycle, the curves shaping it over that time. Sandbox.Mapping.EnvShake.Loop
repeats the cycle until turned off.