getFeedbackFrequency returns the feedback polling frequency [Hz]
 
    getFeedbackFrequency() returns the request rate in Hz for
    feedback a group. 
 
    The default feedback frequency for a new group is 100 Hz. A 
    rate of 0.0 indicates that no feedback is being requested.
 
    Note that the scheduler will try it's best effort to reach
    the set frequency, but the actual arriving rate may be 
    lower as it is limited by the underlying operating system 
    as well as potential hardware constraints.
 
    Example
       % Estimate the 'real' incoming feedback frequency
       targetFrequency = 100; % [Hz]
       pauseTime = 10; % [s]
       group.setFeedbackFrequency(targetFrequency);
       group.startLog();
       pause(pauseTime);
       log = group.stopLog();
       realFrequency = length(log.time) / pauseTime;