fix: refactore oaipmh client handling

oaipmh-core/pom.xml

...mh-core/src/main/java/info/textgrid/middleware/.gitignore

.../textgrid/middleware/IdentifierListDelivererAbstract.java

...o/textgrid/middleware/MetadataFormatListDelivererAbs.java

...fo/textgrid/middleware/MetadataFormatListDelivererDH.java

...fo/textgrid/middleware/MetadataFormatListDelivererTG.java

...a/info/textgrid/middleware/OAIPMHElasticSearchClient.java

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

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

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

...ava/info/textgrid/middleware/RecordDelivererAbstract.java

...ava/info/textgrid/middleware/RecordDelivererDATACITE.java

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

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

...info/textgrid/middleware/RecordListDelivererAbstract.java

...info/textgrid/middleware/RecordListDelivererDATACITE.java

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

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

.../main/java/info/textgrid/middleware/SetListDeliverer.java

.../java/info/textgrid/middleware/OAIPMHUtilitiesOnline.java