Figure out why chemicals.Winterfeld_Scriven_Davis is not a perfect match to custom implementation

[jgostick]

They match within 0.1%, but all our other implemented functions match MUCH better, like nearly perfectly. The code in chemicals for this particular function is quite obscure so I can't really compare it side-by-side with ours. Anyway, leaving it as is for now, and noting to take a look later.