Default value for mterm/lterm in MPC

Fixes #86

do_mpc/controller.py

@@ -616,11 +616,18 @@ def set_objective(self, mterm=None, lterm=None):
         if not isinstance(lterm, (casadi.DM, casadi.SX, casadi.MX)):
             raise Exception('lterm must be of type casadi.DM, casadi.SX or casadi.MX. You have: {}.'.format(type(lterm)))
 
-        self.mterm = mterm
+        if mterm is None:
+            self.mterm = DM(0)
+        else:
+            self.mterm = mterm
         # TODO: This function should be evaluated with scaled variables.
         self.mterm_fun = Function('mterm', [_x, _tvp, _p], [mterm])
 
-        self.lterm = lterm
+        if lterm is None:
+            self.lterm = DM(0)
+        else:
+            self.lterm = lterm
+
         self.lterm_fun = Function('lterm', [_x, _u, _z, _tvp, _p], [lterm])
 
         # Check if lterm and mterm use invalid variables as inputs.