Added direct force plugin and fixes in stretching energy

Samo Penic

2019-03-09 2c4278db6ead5c27e30a3000097ed898c968534e

 src/plugins.c

@@ -174,6 +174,9 @@
      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;