add a isNaN function for integers

 - on some compilers, isnan(int) was not defined and would cause compile
   errors
 - now, isnan(const int) and isnan(const unsigned) return false, which is always
   correct
 - everything is const as ***

include/parser.hpp

@@ -39,6 +39,22 @@
 
 enum RunMode { NONE, GPU_THREADED, CPU, GPU_MPI };
 
+/**
+ * @brief this allows the use of isnan() for int
+ * in the template function fileToVector()
+ */
+inline bool isnan(const int i){
+    return false;
+}
+
+/**
+ * @brief this allows the use of isnan() for unsigned
+ * in the template function fileToVector()
+ */
+inline bool isnan(const unsigned int i){
+    return false;
+}
+
 /**
  * @brief Parses a given file(filename) line by line.
  *        Each line should contain just one value