Fixed RunningStatistics and ThresholdedRunningStatistics (fixes #307)

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.