Added direct force plugin and fixes in stretching energy

Samo Penic

2019-03-09 2c4278db6ead5c27e30a3000097ed898c968534e

 src/plugins/pressure.c

@@ -15,17 +15,14 @@

void vm_energy_before_prepare(ts_vesicle *vesicle, ts_vertex *vtx){
   if(vesicle->pswitch == 1){
   ts_double *dvol=(ts_double *)vesicle->plist->pointer->plugin->details->data;
   ts_uint i;
   //printf ("DVOL was %f\n", *dvol);
   *dvol=0;
   for(i=0;i<vtx->tristar_no;i++) *dvol-=vtx->tristar[i]->volume;
   //printf ("DVOL is %f\n", *dvol);

      ts_double *dvol=(ts_double *)vesicle->plist->pointer->plugin->details->data;
      ts_uint i;
      *dvol=0;
      for(i=0;i<vtx->tristar_no;i++) *dvol-=vtx->tristar[i]->volume;
   }
}

ts_double vm_energy_after_execute(ts_vesicle *vesicle, ts_vertex *vtx){
ts_double vm_energy_after_execute(ts_vesicle *vesicle, ts_vertex *vtx, ts_vertex *backup_vertices){
   ts_double delta_energy=0;
   if(vesicle->pswitch == 1){
      ts_double *dvol=(ts_double *)vesicle->plist->pointer->plugin->details->data;