def codepoint_to_char(codepoint)
      if charset.at(codepoint).nil?
        fail NotInCharset, "Codepoint #{codepoint} does not exist in charset"
      end
      charset.at(codepoint)
    end
def codepoint_to_char(codepoint)
      if charset.at(codepoint).nil?
        fail NotInCharset, "Codepoint #{codepoint} does not exist in charset"
      end
      charset.at(codepoint)
    end
Convert an integer into its string representation according to the
 charset. (only one character)

 Example

   charset = SecretSharing::Charset.by_charset_string "abc"
   charset.codepoint_to_char 1
   # => "a"

 @param codepoint [Integer] Codepoint to retrieve the character for
 @return [String] Retrieved character