running_statistics and thresholded_running_statistics now handels masked data.
- If any sliding window contains a masked value the whole window will be masked. Allowing a grid-cell to have some masked value and still searching for a window without masked values.
- The mask will be preserved for the tail and head parts, which are attached to the pre-statistics.
- The post-processing function preserves the mask for the pre-statistics and the head and tail parts. The padding at the ends and the thresholded values are also masked.