Remove 'native data' and 'native units' (#331)

Resolves #331 (closed)

Somehow resolves #850 (closed)

Base/Util/ConversionUtil.h

+//  ************************************************************************************************
+//
+//  BornAgain: simulate and fit reflection and scattering
+//
+//! @file      Base/Util/ConversionUtil.h
+//! @brief     Implements namespace Conversion.
+//!
+//! @homepage  http://www.bornagainproject.org
+//! @license   GNU General Public License v3 or higher (see COPYING)
+//! @copyright Forschungszentrum Jülich GmbH 2023
+//! @authors   Scientific Computing Group at MLZ (see CITATION, AUTHORS)
+//
+//  ************************************************************************************************
+
+#ifndef BORNAGAIN_BASE_UTIL_CONVERSIONUTIL_H
+#define BORNAGAIN_BASE_UTIL_CONVERSIONUTIL_H
+
+namespace Convert::Refl {
+
+double qz2alpha(double lambda, double qz);
+
+double qz2lambda(double alpha, double qz);
+
+double qz(double alpha, double lambda);
+
+} // namespace Convert::Refl
+
+#endif // BORNAGAIN_BASE_UTIL_CONVERSIONUTIL_H