check sign parameter is defined, remove cell parameter

classical/mpliouville.py

@@ -98,6 +98,8 @@ class MPLP:
         assert maxorder > 0
         assert len(coordinate) > 0 and len(momentum) > 0
         assert len(coordinate) == len(momentum)
+        if recursion == 'chebyshev':
+            assert sign is not None
         self.maxorder = maxorder
         self.Q = coordinate
         self.P = momentum
@@ -526,7 +528,7 @@ class MPLPDV:
         self.P = system.p
         self.ff = system.ff
         self.maxorder = maxorder-1
-        self.cell = system.cell
+#        self.cell = system.cell
         self.system = system
         if numeric is not None:
             self.numeric = numeric