Data used to text mixed activity coefficients may be incorrect

[rkingsbury]

It recently came to my attention that some of the activity coefficient values reported by Rodil et al., which we use in our unit tests, were incorrectly reported. See article correction.

We need to double check the data in [test_mixed_electrolyte_activity.py](https://github.com/KingsburyLab/pyEQL/blob/main/tests/test_mixed_electrolyte_activity.py) to see whether any values need to be updated.