merge adaotions to ES6

Jenkinsfile

docs_tgrep/index.rst

oaipmh-core/pom.xml

...c/main/java/info/textgrid/middleware/DARIAHConstants.java

...main/java/info/textgrid/middleware/DublinCoreBuilder.java

.../java/info/textgrid/middleware/DublinCoreFieldLoader.java

.../src/main/java/info/textgrid/middleware/ErrorHandler.java

...a/info/textgrid/middleware/IdentifierListDelivererDC.java

...nfo/textgrid/middleware/IdentifierListDelivererIDIOM.java

...info/textgrid/middleware/MetadataFormatListDeliverer.java

...re/src/main/java/info/textgrid/middleware/OAIPMHImpl.java

...rc/main/java/info/textgrid/middleware/OAIPMHProducer.java

...c/main/java/info/textgrid/middleware/OAIPMHUtilities.java

.../src/main/java/info/textgrid/middleware/OAI_ESClient.java

.../src/main/java/info/textgrid/middleware/OaiPmhClient.java

...main/java/info/textgrid/middleware/RecordDelivererDC.java

...n/java/info/textgrid/middleware/RecordDelivererIDIOM.java

.../java/info/textgrid/middleware/RecordListDelivererDC.java

...va/info/textgrid/middleware/RecordListDelivererIDIOM.java

...main/java/info/textgrid/middleware/RepIdentification.java