Fix issue #576

If a Histogram has no items, return NaN/NODATA for statistics.  Add default values to statistics.Statistics and an EMPTY object
for convenience.  Minor cleanup.
Added test to check medians were return the same between getMedian and generateStatistics
Merge pull request #578 from ejc123/issue_576

Fix issue #576
Merge pull request #579 from lossyrob/median/test

Added test to check medians were return the same between getMedian and g...