Missing META-INF service declarations for authority factories

The core/sis-referencing/src/main/resources/META-INF/services/ directory registers a CRSAuthorityFactory, but not yet any CSAuthorityFactory, DatumAuthorityFactory or CoordinateOperationAuthorityFactory. The later in particular would be useful for an easier access to the following method:

Set<CoordinateOperation> createFromCoordinateReferenceSystemCodes(String sourceCRS, String targetCRS)

The reason why authority factories other than CRSAuthorityFactory are not yet registered is because it is more convenient for Apache SIS to use a single instance for all kind of authority factories (for sharing the cache). This can be done more cleanly in Java 9 using the new ServiceLoader capability. In the meantime, users can get the factory with following code, which is unsafe, but should work at least with SIS 0.8:

CoordinateOperationAuthorityFactory factory = (CoordinateOperationAuthorityFactory) CRS.getAuthorityFactory("EPSG");