Issue with chimera_layout mistaking single tile for multiple tiles.

**Description**
The chimera_layout function uses the networkx function diameter, which can be > 2 for some configurations of our chimera graphs.

**To Reproduce**
The example on the demo page is working:
https://docs.ocean.dwavesys.com/projects/dwave-networkx/en/latest/reference/drawing.html
However the following modification will cause an error in which the code mistakes the connected graph for being from two connected tiles:
`import networkx as nx
import dwave_networkx as dnx
import matplotlib.pyplot as plt

H = nx.Graph()
H.add_nodes_from([0, 1, 4, 5, 6, 7])
H.add_edges_from([(0, 4), (0, 5), (0, 6), (0, 7)])
H.add_edges_from([(1, 4)])
pos = dnx.chimera_layout(H)
G = dnx.chimera_graph(1,1,4)
ind = dnx.find_chimera_indices(G)
dnx.draw_chimera(G)
dnx.draw_chimera(H, node_color='b', node_shape='*', style='dashed', edge_color='b', width=3)
dnx.chimera_layout(H, scale=1., center=None, dim=2)
plt.show()`

There are a few similar issues, so it might be a good idea to check a few different cases.
This one draws as if 2 is 1.
`H.add_nodes_from([0, 2, 4, 5, 6, 7])
H.add_edges_from([(0, 4), (0, 5), (0, 6), (0, 7)])
H.add_edges_from([(2, 4), (2, 5), (2, 6), (2, 7)])`

**Expected behavior**
Partial chimera graphs should draw correctly.

The first example should draw an extra dashed line from node 1 to node 4.

The second example should produce a chimera graph tile with the 0 node and the 2 node fully connected.

**Environment:**
- OS: Windows 10 Pro
- Python version: 3.7.1

**Additional context**
This issue showed up from a user trying to run the example in drawing chimera graph functions:
https://github.com/dwavesystems/dwave_networkx/issues/90