Create a graph if it does not exist, update if otherwise

I frequently find myself writing the following construct, to check if a graph exists or not, to either post or update it:

```python
if graph_id == None:
    graphspace_instance.post_graph(G)
else:   
    graphspace_instance.update_graph(G)
```
It would be helpful to have a built-in wrapper around that performs this check automatically.