Fix velocity Verlet interface

When setting up the NPT integrator, then the VV integrator, the core
integrator remains on NPT. Since NVT and VV are the same, we can set
the core integrator to NVT when the interface is set to VV.

src/python/espressomd/integrate.pyx

@@ -52,6 +52,8 @@ cdef class Integrator:
         self._method = state['_method']
         if self._method == "STEEPEST_DESCENT":
             self.set_steepest_descent(**state['_steepest_descent_params'])
+        elif self._method == "VV":
+            self.set_vv()
         elif self._method == "NVT":
             self.set_nvt()
         elif self._method == "NPT":
@@ -122,6 +124,7 @@ cdef class Integrator:
         Set the integration method to Velocity Verlet.
 
         """
+        integrate_set_nvt()
         self._method = "VV"
 
     def set_nvt(self):