Parse CRS from GML like published by EPSG

Apache SIS 0.7 and later can read a CRS definition in GML, but all components have to be in a single file. But the CRS definitions provided by EPSG use xlink:href instead for referring to definitions in external documents. SIS automatically follows xlink:href when they are in the same document (except for forward references, which are not yet supported), but this is not sufficient for parsing an EPSG definition.

We need to follow xlink:href to external files. This task is described by SIS-387 for the metadata part, but the same issue applies to CRS definitions. This task is needed for OGC Testbed-19 for the definitions of CRS in space, but should have the side-effect of allowing parsing CRS definition provided online by the EPSG registry in GML. So it should not be necessary anymore for users to install the EPSG database if fetching the definition online is acceptable, and they would always get the latest definition from EPSG.