getIndexOfTag returns the frame index of a tagged element
 
    This method returns the index of a tagged body within the
    array of transforms returned by the forward kinematics.
    Tags need to be defined in HRDF and be unique.
    
    Example
        imuIx = kin.getIndexOfTag('imu');
        frames = kin.getFK('output', fbk.position);
        imuFrame = frames(:,:,imuIx);