Fixed index out of bounds bug when MINIMAL is not the first volume output

Former-commit-id: c4fd60ee

code/src/common/io.cpp

+/*!
+ * \file io.cpp
+ * \brief Set of utility io functions for rtns
+ * \author S. Schotthoefer, J. Wolters et al.
+ */
+
 #include "common/io.h"
 #include "common/config.h"
 #include "common/mesh.h"

code/src/solvers/mnsolver.cpp

@@ -206,8 +206,8 @@ void MNSolver::PrepareOutputFields() {
                 _outputFields[idx_group].resize( 1 );
                 _outputFieldNames[idx_group].resize( 1 );
 
-                _outputFields[0][0].resize( _nCells );
-                _outputFieldNames[0][0] = "radiation flux density";
+                _outputFields[idx_group][0].resize( _nCells );
+                _outputFieldNames[idx_group][0] = "radiation flux density";
                 break;
 
             case MOMENTS:

code/src/solvers/pnsolver.cpp

@@ -371,8 +371,8 @@ void PNSolver::PrepareOutputFields() {
                 _outputFields[idx_group].resize( 1 );
                 _outputFieldNames[idx_group].resize( 1 );
 
-                _outputFields[0][0].resize( _nCells );
-                _outputFieldNames[0][0] = "radiation flux density";
+                _outputFields[idx_group][0].resize( _nCells );
+                _outputFieldNames[idx_group][0] = "radiation flux density";
                 break;
 
             case MOMENTS: