feat(mean_field): added initial implementation

note this implementation lacks many tests

libmeanfield/include/xad_promote_polyfill.h

@@ -0,0 +1,11 @@
+#pragma once
+#include <type_traits>
+
+#if defined(_LIBCPP_VERSION)
+namespace std {
+    template <class... Args>
+    struct __promote {
+        using type = double;
+    };
+}
+#endif