Added constant volume plugin (partially)

Samo Penic

2019-03-08 77a2c7de0dd6563a30467fa2e73f02ec9f4c3deb

 src/energy.c

@@ -238,3 +238,7 @@
   return vesicle->tape->F*ddp;      
   
}

void stretchenergy(ts_vesicle *vesicle, ts_triangle *triangle){
   triangle->energy=vesicle->tape->xkA0/2.0*pow((triangle->area/vesicle->tlist->a0-1.0),2);
}