Fix GetEdgeHistogram() for undirected graphs

The Histogram type 'hist_t' should have a 'double' value, so that
counts of 1/2 can be added when it's known things are going to be
counted twice.

(and I don't know what that 'double_t' thing was doing there)

src/graph/graph.hh

@@ -54,9 +54,9 @@ public:
     };
 
     // histogram types
-    typedef std::tr1::unordered_map<double,size_t> hist_t;
+    typedef std::tr1::unordered_map<double,double> hist_t;
     typedef std::tr1::unordered_map<std::pair<double,double>,double, boost::hash<std::pair<double,double> > > hist2d_t;
-    typedef std::tr1::unordered_map<boost::tuple<double,double,double>,double_t, boost::hash<boost::tuple<double,double,double> > > hist3d_t;
+    typedef std::tr1::unordered_map<boost::tuple<double,double,double>,double, boost::hash<boost::tuple<double,double,double> > > hist3d_t;
     typedef std::tr1::unordered_map<double,std::pair<double,double> > avg_corr_t;
 
     typedef boost::variant<degree_t,std::string> deg_t;