Modifying merging condition of gaussian estimators to allow for merging on top of empty estimator

Signed-off-by: Olivier Van Laere <vanlaere@yahoo-inc.com>
Adding observations of zeros

Signed-off-by: Olivier Van Laere <vanlaere@yahoo-inc.com>
Handling null cases

Signed-off-by: Olivier Van Laere <vanlaere@yahoo-inc.com>
Merge pull request #98 from ovlaere/counting-zeros

Counting zeros

Tested locally, passes mvn test. The TravisCI seems to have some problems with gradle.