fix: only compile edm4eic_merge for podio < 0.17.4 (#64)

### Briefly, what does this PR introduce?
With podio 0.17.4, we can't compile emd4eic_merge anymore since it is
reliant on podio/EventStore.h. We have been ignoring the deprecation
warnings about this for half a year now...

This PR makes the compilation of edm4eic_merge conditional on being
before that version. It renames the source and executable to
event_merge.cpp and edm4eic_event_merge to make clear this isn't working
for frames. Code is not removed since an adventurous soul may port this
to frames so we can do background mixing.

### What kind of change does this PR introduce?
- [x] Bug fix (issue: edm4eic doens't compile with podio 0.17.4)
- [ ] New feature (issue #__)
- [ ] Documentation update
- [ ] Other: __

### Please check if this PR fulfills the following:
- [ ] Tests for the changes have been added
- [ ] Documentation has been added / updated
- [ ] Changes have been communicated to collaborators

### Does this PR introduce breaking changes? What changes might users
need to make to their code?
No.

### Does this PR change default behavior?
Yes, edm4eic_merge is renamed, and will not be installed when newer
versions of podio are used.

utils/CMakeLists.txt

@@ -36,40 +36,45 @@ install(TARGETS edm4eic_utils
 
 if(CLI11_FOUND)
 
-  add_executable(edm4eic_merge src/merge.cpp)
-
-  target_compile_features(edm4eic_merge
-    PUBLIC cxx_auto_type
-    PUBLIC cxx_trailing_return_types
-    PUBLIC cxx_std_17
-    PRIVATE cxx_variadic_templates
-    )
-
-  target_compile_options(edm4eic_merge PRIVATE
-    -Wno-extra
-    -Wno-ignored-qualifiers
-    -Wno-overloaded-virtual
-    -Wno-shadow
-    )
-
-  target_include_directories(edm4eic_merge
-    PUBLIC $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}>
-    PUBLIC $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
-    PUBLIC $<INSTALL_INTERFACE:include>
-    )
-
-  target_link_libraries(edm4eic_merge
-    PUBLIC edm4eic
-    PUBLIC EDM4HEP::edm4hep
-    PUBLIC podio::podio podio::podioRootIO
-    PUBLIC ROOT::GenVector ROOT::MathCore)
-
-  install(TARGETS edm4eic_merge
-    EXPORT ${PROJECT_NAME}Targets
-    LIBRARY DESTINATION lib
-    ARCHIVE DESTINATION lib
-    RUNTIME DESTINATION bin
-    INCLUDES DESTINATION include
-    )
+  if(${podio_VERSION} VERSION_LESS 0.17.4)
+
+    # EventStore-based merging
+    add_executable(edm4eic_event_merge src/event_merge.cpp)
+
+    target_compile_features(edm4eic_event_merge
+      PUBLIC cxx_auto_type
+      PUBLIC cxx_trailing_return_types
+      PUBLIC cxx_std_17
+      PRIVATE cxx_variadic_templates
+      )
+
+    target_compile_options(edm4eic_event_merge PRIVATE
+      -Wno-extra
+      -Wno-ignored-qualifiers
+      -Wno-overloaded-virtual
+      -Wno-shadow
+      )
+
+    target_include_directories(edm4eic_event_merge
+      PUBLIC $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}>
+      PUBLIC $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
+      PUBLIC $<INSTALL_INTERFACE:include>
+      )
+
+    target_link_libraries(edm4eic_event_merge
+      PUBLIC edm4eic
+      PUBLIC EDM4HEP::edm4hep
+      PUBLIC podio::podio podio::podioRootIO
+      PUBLIC ROOT::GenVector ROOT::MathCore)
+
+    install(TARGETS edm4eic_event_merge
+      EXPORT ${PROJECT_NAME}Targets
+      LIBRARY DESTINATION lib
+      ARCHIVE DESTINATION lib
+      RUNTIME DESTINATION bin
+      INCLUDES DESTINATION include
+      )
+
+  endif()
 
 endif()