Simplify UGGridGeometry::corner using constexpr if

The UGGrid code dates to before constexpr if existed, and uses
particular "pointless" casts to achieve a similar effect.
Replace some of these casts by constexpr if, which is much easier
to read.

The code is simplified further because UGGrid has recently started
to use FieldVector for point coordinates.  Therefore, componentwise
copying is not necessary anymore.

dune/grid/uggrid/uggridgeometry.cc

@@ -62,13 +62,8 @@ FieldVector<typename GridImp::ctype, coorddim> UGGridGeometry<mydim,coorddim,Gri
 corner(int i) const
 {
   // This geometry is a vertex
-  if (mydim==0) {
-    FieldVector<typename GridImp::ctype, coorddim> result;
-    for (size_t j=0; j<coorddim; j++)
-      // The cast is here to make the code compile even when target_ is not a node
-      result[j] = ((typename UG_NS<coorddim>::Node*)target_)->myvertex->iv.x[j];
-    return result;
-  }
+  if constexpr (mydim==0)
+    return target_->myvertex->iv.x;
 
   // ////////////////////////////////
   //  This geometry is an element
@@ -77,11 +72,8 @@ corner(int i) const
 
   i = UGGridRenumberer<mydim>::verticesDUNEtoUG(i,type());
 
-  FieldVector<typename GridImp::ctype, coorddim> result;
-  for (size_t j=0; j<coorddim; j++)
-    // The cast is here to make the code compile even when target_ is not an element
-    result[j] = UG_NS<coorddim>::Corner(((typename UG_NS<coorddim>::Element*)target_),i)->myvertex->iv.x[j];
-  return result;
+  if constexpr (mydim==coorddim)
+    return UG_NS<coorddim>::Corner(target_,i)->myvertex->iv.x;
 }
 
 template< int mydim, int coorddim, class GridImp>