Threshold filter does not work properly for values below the threshold

[mnaegelin]

Thanks for the awesome package! One small issue I spotted: the threshold_filter function seems to only filter values where the RRI is greater than $\text{local median} + \text{threshold}$, but not where it is smaller than $\text{local median} - \text{threshold}$:

hrv/hrv/filters.py

Line 216 in 3f813f6

if rri[j] > (np.median(rri[slice_]) + threshold):

Only for the values in the very first local window, the filtering happens in both directions:

hrv/hrv/filters.py

Line 222 in 3f813f6

if abs(rri[j] - np.median(rri[slice_])) > threshold: