Inconsistent existence of coordinate systems in node attributes

Graph node attributes contain inconsistent node attributes depending on initialization.
This makes downstream code unnecessarily complex and unreliable

Example:

```
import dwave_networkx as dnx

P = dnx.pegasus_graph(6, coordinates=False)
print(P.nodes[20])

P = dnx.pegasus_graph(6, coordinates=True)
print(P.nodes[0, 0, 4, 0])

P = dnx.pegasus_graph(6, nice_coordinates=True)
print(P.nodes[0, 0, 0, 0, 0])
```

outputs

```
{'pegasus_index': (0, 0, 4, 0)}
{'linear_index': 20}
{'linear_index': 20, 'pegasus_index': (0, 0, 4, 0)}
```