chore: import upstream snapshot with attribution

cpp_easygraph/functions/basic/avg_degree.cpp

@@ -0,0 +1,9 @@
+#include "avg_degree.h"
+
+#include "../../classes/graph.h"
+py::object average_degree(py::object G) {
+    Graph& G_ = G.cast<Graph&>();
+    int n = G_.node.size();
+    int m = G.attr("number_of_edges")().cast<int>();
+    return py::cast(2.0 * m / n);
+}