Catch2: move to submodule and update to v2.13.8 (#386)

* add another `EXTERNAL_CATCH2` option to be able to use system installations.
* turn on Catch2 test discovery when using submodule

.gitmodules

@@ -10,3 +10,6 @@
 [submodule "3rdparty/lexertl14"]
 	path = 3rdparty/lexertl14
 	url = https://github.com/BenHanson/lexertl14
+[submodule "3rdparty/Catch2"]
+	path = 3rdparty/Catch2
+	url = https://github.com/catchorg/Catch2.git

CMakeLists.txt

@@ -7,6 +7,7 @@ set(CMAKE_VERBOSE_MAKEFILE ON)
 
 option(BUILD_BINDINGS "Build the python bindings" ON)
 option(MorphIO_CXX_WARNINGS "Compile C++ with warnings" ON)
+option(EXTERNAL_CATCH2 "Use Catch2 from external source" OFF)
 option(EXTERNAL_HIGHFIVE "Use HighFive from external source" OFF)
 option(EXTERNAL_PYBIND11 "Use pybind11 from external source" OFF)
 option(MORPHIO_TESTS "Build tests" ON)
@@ -65,5 +66,11 @@ install(
 
 if (MORPHIO_TESTS)
   enable_testing()
+  if (EXTERNAL_CATCH2)
+    find_package(Catch2 REQUIRED)
+  else()
+    add_subdirectory(3rdparty/Catch2)
+    include(3rdparty/Catch2/contrib/Catch.cmake)
+  endif()
   add_subdirectory(tests)
 endif()

tests/CMakeLists.txt

@@ -5,7 +5,7 @@ set(TESTS_SRC
         test_mutable_morphology.cpp
         test_vasculature_morphology.cpp
         )
-set(TESTS_LINK_LIBRAIRIES morphio_static HighFive)
+set(TESTS_LINK_LIBRAIRIES morphio_static HighFive Catch2::Catch2)
 
 if(APPLE)
   add_definitions("-DLIBCXX_INSTALL_FILESYSTEM_LIBRARY=YES")
@@ -36,10 +36,17 @@ if (MORPHIO_ENABLE_COVERAGE)
 endif()
 
 target_link_libraries(unittests
-    PRIVATE ${TESTS_LINK_LIBRAIRIES}
+  PRIVATE 
+  ${TESTS_LINK_LIBRAIRIES}
 )
 
 add_test(NAME unittests
          COMMAND unittests
          WORKING_DIRECTORY ${CMAKE_CURRENT_LIST_DIR}
          )
+
+if (NOT EXTERNAL_CATCH2)
+  catch_discover_tests( unittests
+      WORKING_DIRECTORY ${CMAKE_CURRENT_LIST_DIR}
+      )
+endif()