resizes output vector to better handle vectors

enhances MeshFunction to better handle vector variables
which in turn helps general field transfers work with vectors as well

 resolves: libMesh#3714
see also: #26084, #26045

Author: andcarl384

src/mesh/mesh_function.C

@@ -50,7 +50,8 @@ MeshFunction::MeshFunction (const EquationSystems & eqn_systems,
   _vector              (vec),
   _dof_map             (dof_map),
   _system_vars         (std::move(vars)),
-  _out_of_mesh_mode    (false)
+  _out_of_mesh_mode    (false),
+  _new_resize          (0)
 {
 }
 
@@ -67,7 +68,8 @@ MeshFunction::MeshFunction (const EquationSystems & eqn_systems,
   _vector              (vec),
   _dof_map             (dof_map),
   _system_vars         (1,var),
-  _out_of_mesh_mode    (false)
+  _out_of_mesh_mode    (false),
+  _new_resize          (0)
 {
 }
 
@@ -78,7 +80,8 @@ MeshFunction::MeshFunction (const MeshFunction & mf):
   _vector              (mf._vector),
   _dof_map             (mf._dof_map),
   _system_vars         (mf._system_vars),
-  _out_of_mesh_mode    (mf._out_of_mesh_mode)
+  _out_of_mesh_mode    (mf._out_of_mesh_mode),
+  _new_resize          (0)
 {
   // Initialize the mf and set the point locator if the
   // input mf had done so.
@@ -118,8 +121,31 @@ void MeshFunction::init ()
   const MeshBase & mesh = this->_eqn_systems.get_mesh();
   _point_locator = mesh.sub_point_locator();
 
-  // ready for use
-  this->_initialized = true;
+int vec_dim = 0;
+int n_vector_vars = 0;
+int n_scalar_vars = 0;
+
+int total_variables = _dof_map.n_variables();
+const MeshBase & mesh = this->_eqn_systems.get_mesh();
+_point_locator = mesh.sub_point_locator();
+
+for (int i = 0; i < total_variables; ++i) 
+{
+  FEType fe_type = _dof_map.variable_type(i);
+  if (fe_type != SCALAR) 
+  { 
+    n_vector_vars++;
+    int vector_dim = FEInterface::n_vec_dim(mesh, fe_type); 
+    if (vector_dim > vec_dim)
+      vec_dim = vector_dim;
+  } 
+  else 
+    n_scalar_vars++;
+  
+}
+
+// ready for use
+this->_initialized = true;
 }