Fix loading resources in OSGI context

I am upgrading Weasis to dcm4che 3 and I found that some resource files cannot be loaded in an OSGI environment, because the files are only loaded from the class loader of the current thread. As each jar can have a different class loader in OSGI, it would be safer to load the resources with at least the class loader of a class located in the jar.

Thanks for this new version, great job for the implementation of readers, bulk data, sequence...