Update gsdwriter.py

We pass some extra kwargs that the writer doesn't understand sometime like `write_ff`

```
>           saver(filename=filename, structure=structure, **kwargs)
E           TypeError: write_gsd() got an unexpected keyword argument 'write_ff'
```

Author: Mike Henry

mbuild/formats/gsdwriter.py

@@ -11,7 +11,7 @@
 
 def write_gsd(structure, filename, ref_distance=1.0, ref_mass=1.0,
               ref_energy=1.0, rigid_bodies=None, shift_coords=True,
-              write_special_pairs=True):
+              write_special_pairs=True, **kwargs):
     """Output a GSD file (HOOMD v2 default data format).
 
     Parameters