Make updates and indexing more robust

* Make sure that at least one interval is loaded during the update of the Sigel data; this is crucial if the specified interval size (in days) is smaller than the time span between the start date and today (e.g. start is 2014-11-01, today is 2014-11-27, and interval size is 50 days)
* Always stop with the date of today (no future dates), i.e. make last interval a bit longer
* Check size of enriched.out.json before indexing: Minimum size must be passed to main method of Index.java; application will refuse to build the new Elasticsearch index if the output file is smaller than this threshold
* Avoid building an empty index or an index with only a subset of the available data