new convencience function for checking valid IDs (includes unit tests)

src/utils.h

@@ -335,6 +335,14 @@ namespace LAMMPS_NS {
 
     bool is_double(const std::string &str);
 
+    /** Check if string is a valid ID
+     * ID strings may contain only letters, numbers, and underscores.
+     *
+     * \param str string that should be checked
+     * \return true, if string contains valid id, false otherwise */
+
+    bool is_id(const std::string &str);
+
     /** Try to detect pathname from FILE pointer.
      *
      * Currently only supported on Linux, otherwise will report "(unknown)".