Fix python examples (#1096)

Fix a Python example that was out of date.

python/example/single_cell_swc.py

@@ -50,7 +50,7 @@ cell.place('stim_site', arbor.iclamp(8, 1, current=1))
 cell.place('root', arbor.spike_detector(-10))
 
 # Have one compartment between each sample point.
-cell.compartments_on_samples()
+cell.compartments_on_segments()
 
 # Make single cell model.
 m = arbor.single_cell_model(cell)

python/flat_cell_builder.cpp

@@ -82,7 +82,7 @@ public:
 
         cable_distal_segs_.push_back(p);
 
-        return size()-1;
+        return cable_distal_segs_.size()-1;
     }
 
     void add_label(const char* name, const char* description) {