move data save to beginning to avoid confusion

I.py

@@ -19,6 +19,8 @@ else:
         band = RubberBand(N, a=1)
         lengths[i] = band.length/band.a
     
+    np.save("data/lengths.npy", lengths)
+    
 # histogram
 bin_edges = np.arange(-N-1, N+1, 2)
 values, _ = np.histogram(lengths, bins=bin_edges)
@@ -64,5 +66,3 @@ plt.xlim(xlim)
 plt.xlabel("$L/a$")
 plt.ylabel("$\hat{P}(L)/P(L)$")
 plt.show()
-
-np.save("data/lengths.npy", lengths)