interp2d deprecated

Per [https://scipy.github.io/devdocs/tutorial/interpolate/interp_transition_guide.html](url), interp2d is deprecated. This shows up on ionization_balance_class.py line 15. It seems this code is attempting to interpolate logged_gcr across a logarithmically regular grid of temperature and density. So, if I'm not mistaken, the switch is to simply replace interp2d with RectBivariateSpline. I'll create a pull request with this update.