fix taylorvar grads
added thrust to urdf model.  simple openloop animations look good.  lqr has problems, probably due to issue #165
added dircol stub.  it also fails, presumably due to #165.  added random obstacles and collision objects.  its fun to watch
TSRBM is only used to simulate.  added gravity method to RBM.  support Taylorvars in RigidBodyThrust
slider, since the traj opt w/ taylorvar takes *forever*
merge