Initialize GO sources.

def _init_gos(self, go_sources_arg, relationships_arg):
        """Initialize GO sources."""
        # No GO sources provided
        if not go_sources_arg:
            assert self.go2obj_orig, "go2obj MUST BE PRESENT IF go_sources IS NOT"
            self.go_sources = set(self.go2obj_orig)
            self.go2obj = self.go2obj_orig
            sys.stdout.write("**NOTE: {N:,} SOURCE GO IDS\n".format(N=len(self.go_sources)))
            return
        # GO sources provided
        go_sources = self._init_go_sources(go_sources_arg, self.go2obj_orig)
        # Create new go2obj_user subset matching GO sources
        # Fill with source and parent GO IDs and alternate GO IDs
        go2obj_user = {}
        objrel = CurNHigher(relationships_arg, self.go2obj_orig)
        objrel.get_id2obj_cur_n_high(go2obj_user, go_sources)
        # Add additional GOTerm information, if needed for user task
        kws_gos = {k:v for k, v in self.kws.items() if k in self.kws_aux_gos}
        if kws_gos:
            self._add_goterms_kws(go2obj_user, kws_gos)
        self.go_sources = go_sources
        self.go2obj = go2obj_user