[nixio] Create relationships using general function

'create_relationship()' method takes care of both many-to-one and
many-to-many relationship creation.

Author: achilleas-k

neo/io/nixio.py

@@ -292,8 +292,7 @@ def _nix_to_neo_block(self, nix_block):
             newchx.block = neo_block
 
         # create object links
-        neo_block.create_many_to_one_relationship()
-        neo_block.create_many_to_many_relationship()
+        neo_block.create_relationship()
 
         # reset maps
         self._neo_map = dict()