fix: gregorian calendars refactored, fixes datestamp in oaipmh headers

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

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

.../textgrid/middleware/IdentifierListDelivererAbstract.java

...tgrid/middleware/MetadataFormatListDelivererAbstract.java

...fo/textgrid/middleware/MetadataFormatListDelivererTG.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

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

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

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

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

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

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

...va/info/textgrid/middleware/RecordListDelivererIdiom.java

...nfo/textgrid/middleware/RecordListDelivererInterface.java

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

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