Add FsCheck tests for moving minimum and maximum and fix bugs

I introduced one bug in the previous commit (initializing deque with capacity
0 breaks things). This is now fixed using clever initialization.

There was also a bug when taking moving window over a series with missing
values and the entire window contains missing values - see the newly added
test. We need to remove pre-window items from the Deque even when there is
no value in the current element (assuming I understand movingMin correctly!)