Add AcidKinetic for chemical weathering of rock.

Takes parameters described in [1], and uses eq 6.52 as rate.

[1] Bandstra, J. Z., & Brantley, S. L. (2008). Data Fitting Techniques with Applications to Mineral Dissolution Kinetics. In Kinetics of Water-Rock Interaction (pp. 211–257). New York, NY: Springer New York. doi:10.1007/978-0-387-73563-4_6
Rename temp -> T
Extract method for rate and diffRate of AcidKinetic, fix sign
Tidy up