Remove "using namespace" from headers and make single param constructors explicit

include/algorithm/frigioni.h

@@ -6,8 +6,6 @@
 
 #include <utility>
 
-using namespace graph;
-
 namespace algo {
 
 template<typename T>
@@ -15,7 +13,7 @@ class Frigioni : public DecrementalReachability<T> {
 public:
   Frigioni() = default;
 
-  Frigioni(Digraph<T> G) { this->G = G; }
+  explicit Frigioni(graph::Digraph<T> G) { this->G = G; }
 
   // Initialize the decremental maintenance data structure for general graphs
   void init() override;