Make `compute_ishape` more robust

The shapefile reader does not abort with ishape == - 1. So use None instead.

Analysis/plot_traj.py

@@ -15,7 +15,7 @@ def compute_ishape(n, k1, ishape_last):
     
     if ishape > ishape_last[n[0] - k1]:
         # bad input n
-        return - 1
+        return None
     else:
         return ishape