add trait FieldPosition

Move the trait `FieldPosition` out of the numerical cell type namespace
into the trait section.

Author: psychocoderHPC

include/picongpu/traits/FieldPosition.hpp

@@ -0,0 +1,38 @@
+/* Copyright 2013-2018 Axel Huebl, Heiko Burau, Rene Widera
+ *
+ * This file is part of PIConGPU.
+ *
+ * PIConGPU 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.
+ *
+ * PIConGPU 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 PIConGPU.
+ * If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#pragma once
+
+#include "picongpu/simulation_defines.hpp"
+
+namespace picongpu
+{
+
+namespace traits
+{
+    template<
+        typename T_CellType,
+        typename T_Field,
+        uint32_t T_simDim = simDim
+    >
+    struct FieldPosition;
+
+} //namespace traits
+
+}// namespace picongpu