adding spg4 code option

contrib/CMakeLists.txt

@@ -0,0 +1,11 @@
+if(FSFW_ADD_SPG4_PROPAGATOR)
+    target_sources(${LIB_FSFW_NAME} PRIVATE
+        spg4unit.cpp
+    )
+    target_include_directories(${LIB_FSFW_NAME} PRIVATE
+        ${CMAKE_CURRENT_SOURCE_DIR}/spg4
+    )
+    target_include_directories(${LIB_FSFW_NAME} INTERFACE
+        ${CMAKE_CURRENT_SOURCE_DIR}/spg4
+    )
+endif()