replace start id values for cells and surfaces in DAGMC bounding

[jon-proximafusion]

these two magic numbers can be replaced with self.n_cells and self.n_surfaces

https://github.com/openmc-dev/openmc/blob/a7d1ceba3f9fe5e394a49cdf10b724094f39af11/openmc/dagmc.py#L326C9-L326C20

https://github.com/openmc-dev/openmc/blob/a7d1ceba3f9fe5e394a49cdf10b724094f39af11/openmc/dagmc.py#L396