Add a small downstream project for testing

tests/downstream-project-cmake-test/CMakeLists.txt

@@ -0,0 +1,15 @@
+cmake_minimum_required(VERSION 3.14)
+
+project(DownstreamProjectUsingLCIO)
+
+find_package(LCIO REQUIRED)
+
+# Make sure that the LCIO version is set and usableas a version
+if (${LCIO_VERSION} VERSION_GREATER "0.0.0")
+  message(STATUS "Found LCIO version " ${LCIO_VERSION})
+else()
+  message(FATAL_ERROR "Cannot determine LCIO_VERSION")
+endif()
+
+add_executable(lcio_test_program lcio_test_program.cpp)
+target_link_libraries(lcio_test_program PRIVATE LCIO::lcio)

tests/downstream-project-cmake-test/lcio_test_program.cpp

@@ -0,0 +1,22 @@
+#include "EVENT/LCEvent.h"
+#include "IMPL/LCCollectionVec.h"
+#include "IMPL/LCEventImpl.h"
+#include "IMPL/MCParticleImpl.h"
+#include "MT/LCWriter.h"
+
+int main() {
+  auto mcp = new IMPL::MCParticleImpl();
+  mcp->setPDG(11);
+
+  auto coll = new IMPL::LCCollectionVec(EVENT::LCIO::MCPARTICLE);
+  coll->addElement(mcp);
+
+  auto event = new IMPL::LCEventImpl();
+  event->addCollection(coll, "mcps");
+
+  auto writer = MT::LCWriter();
+  writer.open("test.slcio");
+  writer.writeEvent(event);
+
+  return 0;
+}