Add lactation curve graph to /data/dairy

[squeek502]

Unfortunately, most of the primary sources seem to be gated behind a paywall:

• Wood, P. D. P. "Algebraic model of the lactation curve in cattle." (1967): 164-165.
• Wilmink, J. B. M. "Adjustment of test-day milk, fat and protein yield for age, season and stage of lactation." Livestock Production Science 16.4 (1987): 335-348.

Some comparisons of various lactation curve models:

• http://www.journalofdairyscience.org/article/S0022-0302(06)72250-0/abstract
• http://www.ajol.info/index.php/sajas/article/viewFile/71013/59993
• http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1516-35982010000400026
• http://ceur-ws.org/Vol-1152/paper50.pdf

Various other sources:

• https://articles.extension.org/pages/26242/managing-measures-of-feed-costs:-benchmarking-physical-and-economic-feed-efficiency
• http://afsdairy.ca.uky.edu/extension/nutrition/millkingcows/diagnosingnutritionalproblems/monthlymilkandcomposition
• https://aurelienmadouasse.wordpress.com/2011/03/09/using-the-wilmink-function-to-fit-lactation-curves/