Create a header of fundamental files

src/include/types.h

+/* Distributed under the terms of GPLv3 or later. See COPYING for details. */
+
+/*! \file types.h
+ *
+ * \brief Definition of fundamental types in use.
+ */
+
+#ifndef INCLUDE_TYPES_H_
+#define INCLUDE_TYPES_H_
+
+#include <complex.h>
+
+typedef __complex__ double dcomplex;
+
+/*! \brief Get the real part of a complex number.
+ *
+ * \param z: `complex double` The argument of the function.
+ * \return rz: `double` The real part of the argument.
+ */
+double real(dcomplex z) { return __real__ z; }
+
+/*! \brief Get the imaginary part of a complex number.
+ *
+ * \param z: `complex double` The argument of the function.
+ * \return iz: `double` The imaginary part of the argument.
+ */
+double imag(dcomplex z) { return __imag__ z; }
+#endif