Fix qualifiers for node coordinates: member and constructor argument

This bad qualifiers were already noticed with @alexiane

src/mesh/Mesh.hpp

@@ -18,7 +18,7 @@ class Mesh final : public IMesh
 
  private:
   const std::shared_ptr<const Connectivity> m_connectivity;
-  NodeValue<const Rd> m_xr;
+  const NodeValue<const Rd> m_xr;
 
  public:
   PUGS_INLINE
@@ -85,7 +85,7 @@ class Mesh final : public IMesh
   }
 
   PUGS_INLINE
-  Mesh(const std::shared_ptr<const Connectivity>& connectivity, NodeValue<Rd>& xr)
+  Mesh(const std::shared_ptr<const Connectivity>& connectivity, const NodeValue<const Rd>& xr)
     : m_connectivity{connectivity}, m_xr{xr}
   {
     ;