Add a header with constants

CMakeLists.txt

@@ -104,6 +104,11 @@ install(FILES ${CMAKE_CURRENT_SOURCE_DIR}/LICENSE
               ${CMAKE_CURRENT_SOURCE_DIR}/NOTICE
               DESTINATION ${CMAKE_INSTALL_DOCDIR})
 
+
+install(DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/include/
+        DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/edm4hep
+        FILES_MATCHING PATTERN "*.h")
+
 find_package(nlohmann_json 3.10.5)
 
 add_subdirectory(edm4hep)

include/Constants.h

@@ -0,0 +1,28 @@
+/*
+ * Copyright (c) 2014-2023 Key4hep-Project.
+ *
+ * This file is part of Key4hep.
+ * See https://key4hep.github.io/key4hep-doc/ for further info.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+#ifndef EDM4HEP_CONSTANTS_H
+#define EDM4HEP_CONSTANTS_H
+
+#include <string>
+
+namespace edm4hep {
+  inline constexpr std::string_view CellIDEncoding = "CellIDEncoding";
+}
+
+#endif  // EDM4HEP_CONSTANTS_H