Added Header for common forward declarations

include/xerus/forwardDeclarations.h

+// Xerus - A General Purpose Tensor Library
+// Copyright (C) 2014-2019 Benjamin Huber and Sebastian Wolf.
+//
+// Xerus is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published
+// by the Free Software Foundation, either version 3 of the License,
+// or (at your option) any later version.
+//
+// Xerus 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 Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with Xerus. If not, see <http://www.gnu.org/licenses/>.
+//
+// For further information on Xerus visit https://libXerus.org
+// or contact us at contact@libXerus.org.
+
+/**
+ * @file
+ * @brief Header file containingt forward declarations for the most common types.
+ */
+
+#pragma once
+
+namespace xerus {
+    
+	class Tensor;
+    class TensorNetwork;
+    template<bool isOperator> class TTNetwork;
+    typedef TTNetwork<false> TTTensor;
+    typedef TTNetwork<true> TTOperator;
+
+} // End xerus namespace