Fixed bioavailability CRLF problem.

The bioavailability.txt data file appears to have CRLF line endings which were being automagically changed to just CR line endings on Macs when you cloned the repo. This meant that a fresh checkout on a Mac immediately saw this as a "changed file" that wanted to be committed.

This StackOverflow answer (http://stackoverflow.com/a/5009496) suggested that we needed to explicitly mark bioavailability.txt as always having CRLF line endings regardless of the native line endings, and adding that to .gitattributes does appear to fix the problem.

Alternatively we could change the line endings on this file to CR and commit that. I don't know where the file originated and whether changing the line endings would have implications for anyone else, however.