add A2 stub

src/a2.h

@@ -0,0 +1,17 @@
+#pragma once
+
+#include <vector>
+
+#include "bounding_box.h"
+
+namespace freling {
+
+// The normal signature for this function should be:
+// std::vector<BoundingBox> A2(const Frame& regions);
+// However, since we are mocking this algorithm we just need to fulfill this
+// constraint: "We assume for the sake of simplicity, that no bounding boxe
+// in B′ overlaps more than one region in F Regions".
+// To do so, we need to have the original delimitation of regions.
+std::vector<BoundingBox> A2(const std::vector<BoundingBox>& regions);
+
+}  // namespace freling