fix(OAI-PMH): specifiy schema locations for XML validation

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

.../test/java/ch/unige/solidify/test/oai/OAIServiceTest.java

...il/src/main/java/ch/unige/solidify/SolidifyConstants.java