Closes #696 (closed) This merge request is connected with GSOC 2019's project Kea statistics enhancements and enables storing more than one sample. All modifies concern Observation, StatsMgr, stats_mgr_unittest and observation_unittest files. It modifies the way of adding samples and retrieving them as a JSON. It implements age and size limits, by default set the size limit to 20 and age limit is disabled. The feature of changing limits of Observations during running the program will be add in next merge.