Added empty interface for background calculations

Core/Computation/IBackGround.cpp

+// ************************************************************************** //
+//
+//  BornAgain: simulate and fit scattering at grazing incidence
+//
+//! @file      Core/Computation/IBackGround.cpp
+//! @brief     Implements interface IBackGround.
+//!
+//! @homepage  http://www.bornagainproject.org
+//! @license   GNU General Public License v3 or higher (see COPYING)
+//! @copyright Forschungszentrum Jülich GmbH 2015
+//! @authors   Scientific Computing Group at MLZ Garching
+//! @authors   C. Durniak, M. Ganeva, G. Pospelov, W. Van Herck, J. Wuttke
+//
+// ************************************************************************** //
+
+#include "IBackGround.h"
+
+IBackGround::~IBackGround() {}

Core/Computation/IBackGround.h

+// ************************************************************************** //
+//
+//  BornAgain: simulate and fit scattering at grazing incidence
+//
+//! @file      Core/Computation/IBackGround.h
+//! @brief     Defines interface IBackGround.
+//!
+//! @homepage  http://www.bornagainproject.org
+//! @license   GNU General Public License v3 or higher (see COPYING)
+//! @copyright Forschungszentrum Jülich GmbH 2017
+//! @authors   Scientific Computing Group at MLZ Garching
+//! @authors   C. Durniak, M. Ganeva, G. Pospelov, W. Van Herck, J. Wuttke
+//
+// ************************************************************************** //
+
+#ifndef IBACKGROUND_H
+#define IBACKGROUND_H
+
+//! Interface for a simulating the background signal
+//!
+//! @ingroup algorithms_internal
+
+class IBackGround
+{
+public:
+    virtual ~IBackGround();
+};
+
+#endif // IBACKGROUND_H