Fixed indexing bug caused by a numpy deprecation

[dmkriete]

I encountered the same issue with steady-state ionization balances as previously reported. The bug is caused by a deprecation of using non-tuple sequences for multidimensional indexing in numpy. I fixed it by simply casting the index list to a tuple before using it for indexing.

My understanding is that this should still work for older versions of numpy, since multidimensional indexing with tuples has always been supported. But probably a good idea to test this on your machine before merging.

fixes #23