chemdose_f: Calculate new fluoride concentration after dosing alum.
Description
Applies equation of the form: raw_f - A*alum^a*ph ^ b * raw_f^c. There is no published model, so it is
recommended to fit the coefficients with experimental data. When fitting, the following units must be used:
Alum in mg/L as chemical, Fluoride in mg/L, pH in SU. Default coefficients are fit from Sollo et al (1978). This function
outputs a water class object with an updated fluoride concentration (which will be in M, per standard water units).