Working on fixing RigidBodyInertialMeasurementUnit + test:
* output of IMU is currently not right when the IMU is not just rotated about the view axis. Only testing rotations about view axis for now.
* found some numerical issues when using the y-axis as the view axis for a PlanarRigidBodyManipulator. Haven't quite found the root cause yet, but the test passes if I change the model to have joints that rotate about the x-axis and set the view axis to the x-axis.
* reenabled first part of test
* added some comments on 3D rotated IMUs on planar manipulators
* easier way to compute angledot (and would work for view axes that aren't x,y, or z).