Java VolumeDataAxisDescriptor and MetadataKey has public type/category/name members

These should be made private and only have .getXXX() methods so people don't think it is possible to set them and to be consistent with the other APIs. It is weird that VolumeDataAxisDescriptor is done differently than VolumeDataChannelDescriptor (which has getters) and that the VolumeDataChannelDescriptor uses 'm_' prefix for its members while the others do not ('m_' prefix feels a little weird in Java anyway).