message: Units: Add missing `const` modifiers.
  Some member functions weren't marked as const.
  Also remove some unnecessary `inline`s.
  GPC: units-const
diff: @@ -208,12 +208,14 @@ constexpr Length millimeters(float mm) { return Length(mm / 1000); }
  // Multiplying a VolumetricFlow by a Duration (see below) gives you a Volume.
  class VolumetricFlow : public units_detail::ArithScalar<VolumetricFlow, float> {
  public:
  - [[nodiscard]] constexpr float cubic_m_per_sec() { return val_; }
  - [[nodiscard]] constexpr float ml_per_min() {
  + [[nodiscard]] constexpr float cubic_m_per_sec() const { return val_; }
  + [[nodiscard]] constexpr float ml_per_min() const {
  return val_ * 1000.0f * 1000.0f * 60.0f;
  }
  - [[nodiscard]] constexpr float liters_per_sec() { return val_ * 1000.0f; }
  - [[nodiscard]] constexpr float ml_per_sec() { return val_ * 1.0e6f; }
  + [[nodiscard]] constexpr float liters_per_sec() const {
  + return val_ * 1000.0f;
  + }
  + [[nodiscard]] constexpr float ml_per_sec() const { return val_ * 1.0e6f; }
  private:
  constexpr friend VolumetricFlow cubic_m_per_sec(float m3ps);
  @@ -252,8 +254,8 @@ constexpr VolumetricFlow ml_per_sec(float mmps) {
  // Dividing a Volume by a Duration gives you a VolumetricFlow.
  class Volume : public units_detail::ArithScalar<Volume, float> {
  public:
  - [[nodiscard]] constexpr float cubic_m() { return val_; }
  - [[nodiscard]] constexpr float ml() { return val_ * 1000.0f * 1000.0f; }
  + [[nodiscard]] constexpr float cubic_m() const { return val_; }
  + [[nodiscard]] constexpr float ml() const { return val_ * 1000.0f * 1000.0f; }
  private:
  constexpr friend Volume cubic_m(float m3);
  @@ -363,8 +365,8 @@ public:
  constexpr friend Time operator+(const Duration &a, const Time &b);
  constexpr friend Time operator-(const Time &a, const Duration &b);
  constexpr friend Duration operator-(const Time &a, const Time &b);
  - inline Time &operator+=(const Duration &dt) { return *this = *this + dt; }
  - inline Time &operator-=(const Duration &dt) { return *this = *this - dt; }
  + Time &operator+=(const Duration &dt) { return *this = *this + dt; }
  + Time &operator-=(const Duration &dt) { return *this = *this - dt; }
  private:
  constexpr friend Time millisSinceStartup(uint64_t millis);
  