Support more than 1e4 projections

Currently the numeric suffix is hard-coded to four digits: index_zfill4 and

    def format_data_path_for_data(
        self, data_path: str, index: int, volume_basename: str
    ) -> str:
        """
        Return file path to save the frame at `index` of the current volume
        """
        keywords = {
            "index_zfill4": str(index + self.start_index).zfill(4),
    [...]

        keywords = {
            "index_zfill4": "[0-9]{3,4}",

    [...]

Nabu increasingly has to cope with datasets of more than 10k projections. Thus it would be good to either use 5 digits, or have a flexible number of digits (.zfill(n_digits)).

Related: nabu#336 (closed)