Partially resolves failing IMU unit test. 

Twan added some specific rng seeds to show the remaining part of the bug.
Can you please update the variable names / comments on the `quatdot2...` methods?  

For instance, `quatdot2angularvelMatrix` takes `q` (a bad choice!).  I understand now that the matrix is a function of quat and produces the map from quatdot2angularvel, but initially found it jarring to see things like `quatdot2angularvelMatrix(quat)` in `motionSubspace.m`.  

On Aug 10, 2014, at 1:11 AM, Twan Koolen <notifications@github.com> wrote:

> In systems/plants/RigidBodyInertialMeasurementUnit.m:
> 
> >          accel_base = Jdot*qd + J(1:2,:)*qdd;
> >          R_base_to_body = rotmat(-angle);
> >          accel_body = R_base_to_body * accel_base;
> > -        
> > +
> >          y = [ angle; ...
> >            omega_body; ...
> >            accel_body ];
> >        else  % 3D version
> >          quat_body_to_world = rpy2quat(x(4:6));
> I would prefer to just call forwardKin with quaternion output. We could then use quatdot2angularvel to compute omega without having to go through rpydot and its numerical issues.
> 
> —
> Reply to this email directly or view it on GitHub.
>