Use non-deprecated methods for getting libMesh node points

[loganharbour]

Description

The "copy" constructor for Node (see https://github.com/libMesh/libmesh/blob/cc84387bbd96e12d2d12986402ec8bba282fc0c4/include/geom/node.h#L76) is deprecated.

Builds with current libmesh with --disable-deprecated will fail to compile. Get a reference to the node and use operator() (from parent Point) instead.

See these build failures from Salamander (with cardinal and libmesh --disable-deprecated):

/tmp/build/salamander/cardinal/contrib/openmc/src/mesh.cpp: In member function 'virtual openmc::Position openmc::LibMesh::sample_element(int32_t, uint64_t*) const':
/tmp/build/salamander/cardinal/contrib/openmc/src/mesh.cpp:3673:36: error: use of deleted function 'libMesh::Node::Node(const libMesh::Node&)'
 3673 |     auto node_ref = elem.node_ref(i);
      |                                    ^
In file included from /tmp/build/salamander/moose/libmesh/installed/include/libmesh/elem.h:29,
                 from /tmp/build/salamander/moose/libmesh/installed/include/libmesh/dof_map.h:36,
                 from /tmp/build/salamander/cardinal/contrib/openmc/include/openmc/mesh.h:32,
                 from /tmp/build/salamander/cardinal/contrib/openmc/src/mesh.cpp:1:
/tmp/build/salamander/moose/libmesh/installed/include/libmesh/node.h:79:3: note: declared here
   79 |   Node (const Node & n) = delete;
      |   ^~~~
/tmp/build/salamander/cardinal/contrib/openmc/src/mesh.cpp: In member function 'virtual openmc::Position openmc::LibMesh::vertex(int) const':
/tmp/build/salamander/cardinal/contrib/openmc/src/mesh.cpp:3703:47: error: use of deleted function 'libMesh::Node::Node(const libMesh::Node&)'
 3703 |   const auto node_ref = m_->node_ref(vertex_id);
      |                                               ^
/tmp/build/salamander/moose/libmesh/installed/include/libmesh/node.h:79:3: note: declared here
   79 |   Node (const Node & n) = delete;
      |   ^~~~

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 18) on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

[loganharbour]

@pshriwise easy one for ya.

[nuclearkevin]

Thanks @loganharbour! If @pshriwise doesn't have any problems with this, I'll merge it in a day or two.

[pshriwise]

Looks good to me. Thanks @loganharbour!