Time series moving average

Add simple moving average test to check if time series is drifting.

There are more sophisticated tests for time series behaviour, but they are quite complicated and possibly overkill here.

Actually I am not sure if the wrapping of the boolean return value is handled correctly in line 236 of file _action_cpp.pyx.
Do you guys have an expert opinion on that?
I think this could be done in a more efficient way.  

    void shift_right(){
        val_to_add = *m_end
        val_to_remove = *m_begin
        m_running_sum += val_to_add - val_to_remove
        ++m_end
        ++m_begin
    }
    double get_mean(){
        return m_running_sum / m_window_size
    }

This would be more efficient, but you might have to worry about numerical precision.  You could reduce numerical precision by recomputing directly m_running_sum every now and again.