Made explicit cast to arithmetic type const

  /// Explicit cast to an arithmetic type retains the rpart()

  template <class X,

            CPPDUALS_ENABLE_IF(std::is_arithmetic<X>::value && !is_dual<X>::value)>

  explicit operator X () { return X(_real); }

  explicit operator X() const { return X(_real); }

  /// Get the real part.

  T rpart() const { return _real; }