starting CRON task management
DataManager is now a Singleton
src/main/webapp/WEB-INF/cron.xml: add CRON task to notify
Put DataManager in its own namespace
src/main/java/fr/univnantes/atal/poubatal/opendata/DataManager.java: Factorize exception handling
src/main/java/fr/univnantes/atal/poubatal/opendata/DataManager.java: Try to fix a previous error

e.g. something went wrong during the instance initialization, ping timeout, ...
src/main/java/fr/univnantes/atal/poubatal/datastore/User.java: Started User management