introduce descriptor class to describe a 3D vector

GUI/Models/VectorDescriptor.h

+//  ************************************************************************************************
+//
+//  BornAgain: simulate and fit reflection and scattering
+//
+//! @file      GUI/Models/VectorDescriptor.h
+//! @brief     Defines class VectorDescriptor
+//!
+//! @homepage  http://www.bornagainproject.org
+//! @license   GNU General Public License v3 or higher (see COPYING)
+//! @copyright Forschungszentrum Jülich GmbH 2021
+//! @authors   Scientific Computing Group at MLZ (see CITATION, AUTHORS)
+//
+//  ************************************************************************************************
+
+#ifndef BORNAGAIN_GUI_MODELS_VECTORDESCRIPTOR_H
+#define BORNAGAIN_GUI_MODELS_VECTORDESCRIPTOR_H
+
+#include "GUI/Models/DoubleDescriptor.h"
+#include "GUI/Models/VectorItem.h"
+
+//! Describes properties of a 3D vector, consisting of three double values.
+//!
+//! By using this class, the underlying data scheme is hidden from the user of the data. This e.g.
+//! eases SessionItem migration. The underlying implementation can be a VectorItem, a member array
+//! of three doubles, three double members, or any other construction to hold the three values.
+class VectorDescriptor {
+public:
+    //! Operates on a VectorItem.
+    //!
+    //! The settings (like decimals, limits) are taken from the item.
+    //! Only for easier migration. Should be removed after SessionItem refactoring.
+    VectorDescriptor(VectorItem* item, const variant<QString, Unit>& unit)
+        : x(item->xItem(), unit), y(item->yItem(), unit), z(item->zItem(), unit)
+    {
+    }
+    DoubleDescriptor x;
+    DoubleDescriptor y;
+    DoubleDescriptor z;
+};
+
+#endif // BORNAGAIN_GUI_MODELS_VECTORDESCRIPTOR_H