Add OndselSolver as git submodule

also update github actions to pull submodules

src/3rdParty/CMakeLists.txt

@@ -4,4 +4,12 @@ if (BUILD_SMESH AND NOT FREECAD_USE_EXTERNAL_SMESH)
 endif()
 
 add_subdirectory(lazy_loader)
-add_subdirectory(libE57Format)
+add_subdirectory(libE57Format)
+
+if (BUILD_ASSEMBLY AND NOT FREECAD_USE_EXTERNAL_ONDSELSOLVER)
+    if( NOT EXISTS "${CMAKE_SOURCE_DIR}/src/3rdParty/OndselSolver/CMakeLists.txt" )
+        message( SEND_ERROR "The OndselSolver git submodule is not available. Please run
+        git submodule update --init" )
+    endif()
+    add_subdirectory(OndselSolver)
+endif()