use a more typical extension for C++ header files

Makefile

@@ -1,6 +1,6 @@
 default: dashing
 
-dashing: main.cc dashing.cc dashing.h
+dashing: main.cc dashing.cc dashing.hh parse_numbers.hh
 	g++ -W -Wall -O2 -g -std=c++11 $(filter %.cc, $^) -o $@ -lboost_random
-dashing-noopt: main.cc dashing.cc dashing.h
+dashing-noopt: main.cc dashing.cc dashing.hh parse_numbers.hh
 	g++ -W -Wall -O0 -g -std=c++11 $(filter %.cc, $^) -o $@ -lboost_random

dashing.cc

@@ -1,5 +1,5 @@
-#include "parse_numbers.h"
-#include "dashing.h"
+#include "parse_numbers.hh"
+#include "dashing.hh"
 namespace dashing
 {
 std::vector<double> parse_numbers(std::string line) {

main.cc

@@ -20,8 +20,8 @@ freely, subject to the following restrictions:
 
 // demo and benchmark program for dashing
 
-#include "dashing.h"
-#include "parse_numbers.h"
+#include "dashing.hh"
+#include "parse_numbers.hh"
 #include <iostream>
 #include <boost/random.hpp>
 #include <boost/random/random_device.hpp>