fixed compiler error under ubuntu 16.04

bhtree_mpi/src/datastructures/Body.cpp

@@ -9,6 +9,7 @@
 #include <cstring>
 #include <cstdio>
 #include <algorithm>
+#include <cmath>
 
 namespace nbody {
 	using namespace std;
@@ -72,7 +73,7 @@ namespace nbody {
 	}
 
 	bool validDouble(double val) {
-		return !::isnan(val) && isfinite(val);
+		return !isnan(val) && isfinite(val);
 	}
 
 	bool validBody(Body body) {