Add temperature and concentration pKa correction constants

[dantruitt]

Since pKa will vary with both the concentration of a species, as well as the temperature of the solution, I think it will be worthwhile to introduce a method to calculate a more precise pKa value for a species given one or more constants defining the relationship between a species' dissociation constant and temperature and concentration.

I will hopefully find some time to implement and create a PR in the next week or two.

[dantruitt]

Initial commit is up here: https://github.com/dantruitt/pHcalc/tree/temp-correction

These changes anecdotally improve the accuracy of the model when comparing predicted values with values measured after mixing buffers, but I'm not personally satisfied that the ion charge values I used for ionic activity, or even the principle of using ionic activity to approximate an "effective" pKa, are totally scientifically sound.