vtkCellGrid: Move definitions of non-templated functions to CXX file

- This commit refactors the code to resolve `-Wunique-object-duplication`.
- While in here, also moves the implementation of the other non templated `std::vector<std::string> vtkCellGrid::GetCellTypes()` function to `vtkCellGrid.cxx` for consistency.

Common/DataModel/vtkCellGrid.cxx

@@ -422,6 +422,34 @@ int vtkCellGrid::RemoveUnusedCellMetadata()
   return numRemoved;
 }
 
+std::vector<vtkStringToken> vtkCellGrid::CellTypeArray() const
+{
+#if defined(_MSC_VER) && _MSC_VER >= 1930 && _MSC_VER < 1940 /*17.4+*/
+  // MSVC 2022 shows LNK1161 when an exported method uses thread_local in its implementation.
+  // See https://github.com/pytorch/pytorch/issues/87957 for more. We omit the
+  // thread_local here, which makes this method non-threadsafe on Windows, which
+  // should be OK in most cases.
+  static std::vector<vtkStringToken> cellTypes;
+#else
+  static thread_local std::vector<vtkStringToken> cellTypes;
+#endif
+  cellTypes.clear();
+  this->CellTypes(cellTypes);
+  return cellTypes;
+}
+
+std::vector<std::string> vtkCellGrid::GetCellTypes() const
+{
+  auto cta = this->CellTypeArray();
+  std::vector<std::string> result;
+  result.reserve(cta.size());
+  for (const auto& cellTypeToken : cta)
+  {
+    result.push_back(cellTypeToken.Data());
+  }
+  return result;
+}
+
 const vtkCellMetadata* vtkCellGrid::GetCellType(vtkStringToken cellTypeName) const
 {
   auto it = this->Cells.find(cellTypeName);

Common/DataModel/vtkCellGrid.h

@@ -282,32 +282,8 @@ public:
     this->CellTypes(cellTypes);
     return cellTypes;
   }
-  std::vector<vtkStringToken> CellTypeArray() const
-  {
-#if defined(_MSC_VER) && _MSC_VER >= 1930 && _MSC_VER < 1940 /*17.4+*/
-    // MSVC 2022 bombs when an exported method uses thread_local in its implementation.
-    // See https://github.com/pytorch/pytorch/issues/87957 for more. We omit the
-    // thread_local here, which makes this method non-threadsafe on Windows, which
-    // should be OK in most cases.
-    static std::vector<vtkStringToken> cellTypes;
-#else
-    static thread_local std::vector<vtkStringToken> cellTypes;
-#endif
-    cellTypes.clear();
-    this->CellTypes(cellTypes);
-    return cellTypes;
-  }
-  std::vector<std::string> GetCellTypes() const
-  {
-    auto cta = this->CellTypeArray();
-    std::vector<std::string> result;
-    result.reserve(cta.size());
-    for (const auto& cellTypeToken : cta)
-    {
-      result.push_back(cellTypeToken.Data());
-    }
-    return result;
-  }
+  std::vector<vtkStringToken> CellTypeArray() const;
+  std::vector<std::string> GetCellTypes() const;
   ///@}
 
   ///@{