Simplify header includes.

#include <thread>

#include <random>

#include "../graph_framework/backend.hpp"

#include "../graph_framework/solver.hpp"

#include "../graph_framework/timing.hpp"

#define arithmetic_h

#include "piecewise.hpp"

#include "register.hpp"

namespace graph {

//------------------------------------------------------------------------------

#ifndef backend_h

#define backend_h

#include <complex>

#include <algorithm>

#include <vector>

#include "register.hpp"

#ifndef cpu_context_h

#define cpu_context_h

#include <iostream>

#include <fstream>

#include <cstdlib>

#include <map>

#include <dlfcn.h>

#ifndef cuda_context_h

#define cuda_context_h

#include <vector>

#include <array>

#include <cuda.h>