add star_subst() utility function that replaces a '*' in a string with a number

src/utils.h

@@ -368,13 +368,27 @@ namespace utils {
 
   std::string trim(const std::string &line);
 
-  /*! Return string with anything from '#' onward removed
+  /*! Return string with anything from the first '#' character onward removed
    *
    * \param line  string that should be trimmed
    * \return new string without comment (string) */
 
   std::string trim_comment(const std::string &line);
 
+  /*! Replace first '*' character in a string with a number, optionally zero-padded
+   *
+   * If there is no '*' character in the string, return the original string.
+   * If the number requires more characters than the value of the *pad*
+   * argument, do not add zeros; otherwise add as many zeroes as needed to
+   * the left to make the the number representation *pad* characters wide.
+   *
+   * \param name  string with file containing a '*' (or not)
+   * \param step  step number to replace the (first) '*'
+   * \param pad   zero-padding (may be zero)
+   * \return  processed string */
+
+  std::string star_subst(const std::string &name, bigint step, int pad);
+
   /*! Check if a string will likely have UTF-8 encoded characters
    *
    * UTF-8 uses the 7-bit standard ASCII table for the first 127 characters and