clean up the licensing preable of source files

the in-file lists of authors has been removed in favor of a global
list of authors in the LICENSE file. this is done because (a)
maintaining a list of authors at the beginning of a file is a major
pain in the a**, (b) the list of authors was not accurate in about 85%
of all cases where more than one person was involved and (c) this list
is not legally binding in any way (the copyright is at the person who
authored a given change, if these lists had any legal relevance, one
could "aquire" the copyright of the module by forking it and removing
the lists...)

the only exception of this is the eWoms fork of dune-istl's solvers.hh
file. This is beneficial because the authors of that file do not
appear in the global list. Further, carrying the fork of that file is
required because we would like to use a reasonable convergence
criterion for the linear solver. (the solvers from dune-istl do
neither support user-defined convergence criteria not do the
developers want support for it. (my patch was rejected a few years
ago.))

examples/problems/fingerproblem.hh

@@ -1,8 +1,6 @@
 // -*- mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*-
 // vi: set et ts=4 sw=4 sts=4:
 /*
-  Copyright (C) 2008-2013 by Andreas Lauser
-
   This file is part of the Open Porous Media project (OPM).
 
   OPM is free software: you can redistribute it and/or modify
@@ -17,6 +15,10 @@
 
   You should have received a copy of the GNU General Public License
   along with OPM.  If not, see <http://www.gnu.org/licenses/>.
+
+  Consult the COPYING file in the top-level source directory of this
+  module for the precise wording of the license and the list of
+  copyright holders.
 */
 /*!
  * \file