fix: refactor some of the calendar methods, add tests

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

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

.../textgrid/middleware/IdentifierListDelivererAbstract.java

...nfo/textgrid/middleware/IdentifierListDelivererIdiom.java

...e/src/main/java/info/textgrid/middleware/IdiomImages.java

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

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

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

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

...ava/info/textgrid/middleware/RecordDelivererDatacite.java

...n/java/info/textgrid/middleware/RecordDelivererIdiom.java

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

...va/info/textgrid/middleware/test/TestOaipmhUtilities.java

...va/info/textgrid/middleware/test/TestTGOaipmhLocally.java

...extgrid/middleware/test/online/OaipmhUtilitiesOnline.java

...grid/middleware/test/online/tg/TestTGGetRecordOnline.java

...iddleware/test/online/tg/TestTGListIdentifiersOnline.java

...id/middleware/test/online/tg/TestTGListRecordsOnline.java