[API] Remove dependency resolution from Singleton

It is not actually used anywhere (since the removal of the old pe), is very hard to read, and essentially contains a custom clone of std::tuple.
If it is ever needed, it could easily be re-implemented using std::tuple and variadic templates in significantly less code.

src/core/singleton/NullType.h

-//======================================================================================================================
-//
-//  This file is part of waLBerla. waLBerla is free software: you can 
-//  redistribute it and/or modify it under the terms of the GNU General Public
-//  License as published by the Free Software Foundation, either version 3 of 
-//  the License, or (at your option) any later version.
-//  
-//  waLBerla is distributed in the hope that it will be useful, but WITHOUT 
-//  ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 
-//  FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License 
-//  for more details.
-//  
-//  You should have received a copy of the GNU General Public License along
-//  with waLBerla (see COPYING.txt). If not, see <http://www.gnu.org/licenses/>.
-//
-//! \file NullType.h
-//! \ingroup core
-//! \author Klaus Iglberger
-//! \author Florian Schornbaum <florian.schornbaum@fau.de>
-//
-//======================================================================================================================
-
-#pragma once
-
-
-
-///\cond internal
-namespace walberla {
-namespace singleton {
-
-
-struct NullType{};
-
-
-} // namespace singleton
-} // namespace walberla
-///\endcond