add header file to store physical constants.

Former-commit-id: 98d7363c1f34994b79b4d59eadd6561b3600b423

src/const.h

+/*******************************************************************************
+ * This file is part of GadgetSMP.
+ * Coypright (c) 2012 Pedro Gonnet (pedro.gonnet@durham.ac.uk)
+ * 
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published
+ * by the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ * 
+ * This program 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 Lesser General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ * 
+ ******************************************************************************/
+
+
+/* Physical constants. */
+#define const_gamma (5.0f/3.0f)