Fixing a linking error for chemistry with no network

The original code caused a linking error of outputs.cpp and history.cpp with icpc when compiled with -O0 because ChemNetwork::species_names (a static const member) was declared but not set. The error does not appear with -O3, possibly because it is optimized out. (I do not know why gcc has no problem with this.)

This fix adds none.cpp for an empty chemical network and initialize species_names.

Author: tomidakn

configure.py

@@ -722,7 +722,7 @@ def c_to_cpp(arg):
 else:
     definitions['CHEMISTRY_ENABLED'] = '0'
     definitions['NUMBER_CHEMICAL_SPECIES'] = '0'
-    makefile_options['CHEMNET_FILE'] = ''
+    makefile_options['CHEMNET_FILE'] = 'src/chemistry/network/none.cpp'
     definitions['CHEMNETWORK_HEADER'] = '../chemistry/network/chem_network.hpp'
 
 # check number of species and scalars

src/chemistry/network/none.cpp

@@ -0,0 +1,16 @@
+//========================================================================================
+// Athena++ astrophysical MHD code
+// Copyright(C) 2014 James M. Stone <jmstone@princeton.edu> and other code contributors
+// Licensed under the 3-clause BSD License, see LICENSE file for details
+//========================================================================================
+//! \file none.cpp
+//! \brief implementation of empty (default) chemical network
+
+// Athena++ header
+#include "../../defs.hpp"
+#include "chem_network.hpp"
+#include "network.hpp"
+
+// species names
+const std::array<std::string, NSPECIES> ChemNetwork::species_names = {};
+