STYLE: Add SPDX license identifiers to all ITK source files

Add machine-readable SPDX headers to all ITK-owned source files
following VTK's convention: two // (or #) comment lines before
the existing license block.

  // SPDX-FileCopyrightText: Copyright NumFOCUS
  // SPDX-License-Identifier: Apache-2.0

This completes Phase 2 of the SBOM roadmap (issue #4302).
Applied by Utilities/Maintenance/AddSPDXHeaders.py.
5503 files modified. ThirdParty code is excluded.

Author: hjmjohnson

Examples/DataRepresentation/Image/Image1.cxx

@@ -1,3 +1,5 @@
+// SPDX-FileCopyrightText: Copyright NumFOCUS
+// SPDX-License-Identifier: Apache-2.0
 /*=========================================================================
  *
  *  Copyright NumFOCUS

Examples/DataRepresentation/Image/Image2.cxx

@@ -1,3 +1,5 @@
+// SPDX-FileCopyrightText: Copyright NumFOCUS
+// SPDX-License-Identifier: Apache-2.0
 /*=========================================================================
  *
  *  Copyright NumFOCUS

Examples/DataRepresentation/Image/Image3.cxx

@@ -1,3 +1,5 @@
+// SPDX-FileCopyrightText: Copyright NumFOCUS
+// SPDX-License-Identifier: Apache-2.0
 /*=========================================================================
  *
  *  Copyright NumFOCUS

Examples/DataRepresentation/Image/Image4.cxx

@@ -1,3 +1,5 @@
+// SPDX-FileCopyrightText: Copyright NumFOCUS
+// SPDX-License-Identifier: Apache-2.0
 /*=========================================================================
  *
  *  Copyright NumFOCUS

Examples/DataRepresentation/Image/Image5.cxx

@@ -1,3 +1,5 @@
+// SPDX-FileCopyrightText: Copyright NumFOCUS
+// SPDX-License-Identifier: Apache-2.0
 /*=========================================================================
  *
  *  Copyright NumFOCUS

Examples/DataRepresentation/Image/ImageAdaptor1.cxx

@@ -1,3 +1,5 @@
+// SPDX-FileCopyrightText: Copyright NumFOCUS
+// SPDX-License-Identifier: Apache-2.0
 /*=========================================================================
  *
  *  Copyright NumFOCUS

Examples/DataRepresentation/Image/ImageAdaptor2.cxx

@@ -1,3 +1,5 @@
+// SPDX-FileCopyrightText: Copyright NumFOCUS
+// SPDX-License-Identifier: Apache-2.0
 /*=========================================================================
  *
  *  Copyright NumFOCUS

Examples/DataRepresentation/Image/ImageAdaptor3.cxx

@@ -1,3 +1,5 @@
+// SPDX-FileCopyrightText: Copyright NumFOCUS
+// SPDX-License-Identifier: Apache-2.0
 /*=========================================================================
  *
  *  Copyright NumFOCUS

Examples/DataRepresentation/Image/ImageAdaptor4.cxx

@@ -1,3 +1,5 @@
+// SPDX-FileCopyrightText: Copyright NumFOCUS
+// SPDX-License-Identifier: Apache-2.0
 /*=========================================================================
  *
  *  Copyright NumFOCUS

Examples/DataRepresentation/Image/ImageToArray.py

@@ -1,3 +1,5 @@
+# SPDX-FileCopyrightText: Copyright NumFOCUS
+# SPDX-License-Identifier: Apache-2.0
 # ==========================================================================
 #
 #   Copyright NumFOCUS