Remove gpu stuff from csg.hpp

#ifndef CSG_H_

#define CSG_H_

#if defined(CSG_USE_GPU) && !defined(CSG_GPU_HOST_DEVICE)

#define CSG_GPU_HOST_DEVICE __host__ __device__

#else

#define CSG_GPU_HOST_DEVICE

#endif

#include <array>

#include <memory>

#include <tuple>

namespace csg {

struct GPUable {};

template <class D, class Enable = void> struct IsGPUable : std::false_type {};

template <class D>

struct IsGPUable<

    D, typename std::enable_if<std::is_base_of<GPUable, D>::value>::type>

    : std::true_type {};

struct Tree;

std::shared_ptr<Tree> parse_csg(std::string);

class CsgIF : GPUable {

class CsgIF {

public:

  CsgIF(std::shared_ptr<Tree> a_state, bool is_internal_flow = false);

  CsgIF(const CsgIF &rhs);

  CsgIF &operator=(CsgIF &&rhs) = delete;

  ~CsgIF();

  CSG_GPU_HOST_DEVICE

  double operator()(double xx, double yy, double zz) const noexcept;

  inline double operator()(const std::array<double, 3> &p) const noexcept {