feat(OAI-PMH): output XML namespaces at metadata level by default

...i-model/src/main/java/ch/unige/solidify/OAIConstants.java

...-model/src/main/java/ch/unige/solidify/OAIProperties.java

...n/java/ch/unige/solidify/OAIXmlNamespacePrefixMapper.java

...ain/java/ch/unige/solidify/model/oai/MetadataContent.java

...n/java/ch/unige/solidify/model/oai/OAIRepositoryInfo.java

.../unige/solidify/controller/oai/OAIProviderController.java

...h/src/main/java/ch/unige/solidify/service/OAIService.java