Return a string containing the *char_count* bytes at the file
        position determined by self._base_offset + *base* + *offset*.

def read_str(self, char_count, base, offset=0):
        """
        Return a string containing the *char_count* bytes at the file
        position determined by self._base_offset + *base* + *offset*.
        """
        def str_struct(char_count):
            format_ = '%ds' % char_count
            return Struct(format_)
        struct = str_struct(char_count)
        chars = self._unpack_item(struct, base, offset)
        unicode_str = chars.decode('UTF-8')
        return unicode_str