Fix get_hgap() for very small networks

analyze.py

@@ -217,8 +217,12 @@ def get_hgap(g):
     g = GraphView(g, directed=False)
     remove_parallel_edges(g)
     T = hashimoto(g, compact=True, operator=True)
-    ew = scipy.sparse.linalg.eigs(T, k=1, which="LR", return_eigenvectors=False)
-    g.clear()
+    try:
+        ew = scipy.sparse.linalg.eigs(T, k=1, which="LR", return_eigenvectors=False)
+    except TypeError:
+        return numpy.nan
+    finally:
+        g.clear()
     return float(ew.real[0])
 
 @register("diameter", "(Pseudo-) diameter")