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

include/algorithm/decremental_reachability.h

@@ -8,7 +8,7 @@ namespace algo {
 template<typename T>
 class DecrementalReachability {
 public:
-  virtual ~DecrementalReachability() {};
+  virtual ~DecrementalReachability() = default;
 
   // This method is implemented and executed by all roditty and zwick
   // algorithms, it constructs the data structures used in other operations