Added stretching energy plugin

Samo Penic

2019-03-09 4df6ed05246cf8bc9fe0cb0c8766ebc368d2f512

 src/plugins.c

@@ -171,6 +171,12 @@
      if(plist->plugin[i]->function->vm_before_montecarlo_constraint!=NULL){
         plist->chain->vm_before_montecarlo_constraint=plugin_to_chain(plist->chain->vm_before_montecarlo_constraint, plist->plugin[i]);
      }
      if(plist->plugin[i]->function->vm_new_state_accepted!=NULL){
         plist->chain->vm_new_state_accepted=plugin_to_chain(plist->chain->vm_new_state_accepted, plist->plugin[i]);
      }
      if(plist->plugin[i]->function->vm_new_state_rejected!=NULL){
         plist->chain->vm_new_state_rejected=plugin_to_chain(plist->chain->vm_new_state_rejected, plist->plugin[i]);
      }
   }
   plist->n=number_of_plugins;
   return plist;