Added function that computes standard error of the mean.

(this is for practice. Ok to delete)
Add vulnerability but doesnt work yet

As far as I can tell, vulnerability should be correctly calculated in the C++ package, but I’m having trouble making it communicate with test-param-manipulate.r I also added a line of code for producing the vulnerability figure as an option - but this requires commenting out the density plot. I figure we’ll make a fancy 2 panel plot, but didn’t want to tackle par. Sean: any idea why vuln doesn’t work?