Add support for elasticsearch 6

[damian0604]

As @FeLoe has researched, this will be a major operation (see also https://www.elastic.co/guide/en/elasticsearch/reference/current/removal-of-types.html ). We basically need to completely remove the concept of 'doctypes' and need to see them as a 'normal' field, requiring us to re-write large parts of the code (e.g., to get an overview of doctypes)

[FeLoe]

Branch elasticsearch6 now ready for testing!