bugfix: minimize the index size for small d-graph values

deconvolution/dgraph/VariableDGIndex.py

@@ -17,6 +17,6 @@ class VariableDGIndex(AbstractDGIndex):
     def add_dgraph(self, dg):
         barcodes = dg.node_set
 
-        for size in range(len(barcodes)-self.size, len(barcodes)+1):
+        for size in range(max(1, len(barcodes)-self.size), len(barcodes)+1):
             for tup in combinations(barcodes, size):
                 self.add_value(frozenset(tup), dg)