Adding an origin attribute to VOSI tables coming from RegistryResource.

This points back to the parent instance.

This is an attempt to address
astropy#289 (comment)

pyvo/registry/regtap.py

@@ -755,6 +755,9 @@ def _build_vosi_table(self, table_row, columns):
         res._columns = [
             self._build_vosi_column(row)
             for row in columns]
+
+        res.origin = self
+
         return res
 
     def get_tables(self, table_limit=20):
@@ -763,7 +766,8 @@ def get_tables(self, table_limit=20):
 
         This returns a dict with table names as keys and vosi.Table
         objects as values (pretty much what tables returns for a TAP
-        service).
+        service).  The table instances will have an ``origin`` attribute
+        pointing back to the registry record.
 
         Note that not only TAP services can (and do) define table
         structures.  The meaning of non-TAP tables is not always

pyvo/registry/tests/test_regtap.py

@@ -611,6 +611,9 @@ def test_get_tables_table_instance(self, flash_tables):
         assert (flash_tables["flashheros.data"].title
             == "Flash/Heros SSA table")
 
+        assert (flash_tables["flashheros.data"].origin.ivoid
+            == "ivo://org.gavo.dc/flashheros/q/ssa")
+
     @pytest.mark.remote_data
     def test_get_tables_column_meta(self, flash_tables):
         def getflashcol(name):