add support for fixed string arrays

[sophiamaedler]

currently the memory mapped temp arrays only work with arrays of numeric types
This adds support for fixed string types.

For e.g.:

#create an empty memory mapped array
dt = np.dtype(f'S{fixed_length}')
self._tmp_single_cell_index_path = create_empty_mmap(
                                        shape=single_cell_index_shape,
                                        dtype=dt,
                                        tmp_dir_abs_path=self._tmp_dir_path,
                                    )

#reconnect to the array (potentially from a different process)
 _tmp_single_cell_index = mmap_array_from_path(self._tmp_single_cell_index_path)

#add results to the array
_tmp_single_cell_index[save_index] = [save_index, cell_id]. #typing is enforced automatically here no need to convert to fixed string before hand