Add is_positive_num() to UtilityClass.cpp

This is a convenience function to tell if a string is not only a number
(and can thus be passed into atoi()), but is also positive in
particular.

desktop_version/src/UtilityClass.h

@@ -11,6 +11,8 @@ std::vector<std::string> split(const std::string &s, char delim, std::vector<std
 
 std::vector<std::string> split(const std::string &s, char delim);
 
+bool is_positive_num(const std::string& str);
+
 
 //helperClass
 class UtilityClass