<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def gauss_fltr_opencv(dem, size=3, sigma=1):
 """OpenCV Gaussian filter Still propagates NaN values """

    import cv2
    dem = malib.checkma(dem)
    dem_cv = cv2.GaussianBlur(dem.filled(np.nan), (size, size), sigma)
    out = np.ma.fix_invalid(dem_cv)
    out.set_fill_value(dem.fill_value)
    return out