Check the convergence of the ion population solver

[chvogl]

Is your feature request related to a problem? Please describe. Currently, the ion population solver stops if the fractional change of the electron density between iterations is less than 5 %: https://github.com/tardis-sn/tardis/blob/ea86abf1e180d0cb203b6cb3c2b4aada87d0c174/tardis/plasma/properties/ion_population.py#L335 This does not guarantee that the resulting electron densities are accurate to less than 5 % if the convergence is slow.

Describe the solution you'd like We should investigate if different threshold values for the convergence (n_e_convergence_threshold) lead to differences in the ion populations. Generally, I think we should use a lower value than the currently used 5 %.