add an example for tasklets

it is IMO not good as a unit test because it does not excercise the
mechanism at all, but it shows how the mechanism is supposed to be
used.

tests/models/test_tasklets.cpp

@@ -0,0 +1,81 @@
+// -*- mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*-
+// vi: set et ts=4 sw=4 sts=4:
+/*
+  This file is part of the Open Porous Media project (OPM).
+
+  OPM 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 2 of the License, or
+  (at your option) any later version.
+
+  OPM 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 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
+ *
+ * \brief This file serves as an example of how to use the tasklet mechanism for
+ *        asynchronous work.
+ */
+#include "config.h"
+
+#include <ewoms/parallel/tasklets.hh>
+
+#include <chrono>
+#include <iostream>
+
+class SleepTasklet : public Ewoms::TaskletInterface
+{
+public:
+    SleepTasklet(int mseconds)
+        : mseconds_(mseconds)
+    {
+        n_ = numInstantiated_;
+        ++ numInstantiated_;
+    }
+
+    void run()
+    {
+        std::this_thread::sleep_for(std::chrono::milliseconds(mseconds_));
+        std::cout << "Sleep tasklet " << n_ << " of " << mseconds_ << " ms finished" << std::endl;
+    }
+
+private:
+    static int numInstantiated_;
+    int n_;
+    int mseconds_;
+};
+
+int SleepTasklet::numInstantiated_ = 0;
+
+int main()
+{
+    Ewoms::TaskletRunner tr(2);
+
+    for (int i = 0; i < 5; ++ i) {
+        //auto st = std::make_shared<SleepTasklet>((i + 1)*1000);
+        auto st = std::make_shared<SleepTasklet>(100);
+        tr.dispatch(st);
+    }
+
+    std::cout << "before barrier" << std::endl;
+    tr.barrier();
+    std::cout << "after barrier" << std::endl;
+
+    for (int i = 0; i < 7; ++ i) {
+        auto st = std::make_shared<SleepTasklet>(500);
+        tr.dispatch(st);
+    }
+
+    return 0;
+}
+