censored weibull function about math

[syclik]

From @bob-carpenter on December 4, 2014 2:52

I have to add right away that I'm not a huge fan of this kind of function that overloads y as either a censoring bound or an outcome. I'd prefer

y_obs ~ weibull(alpha,sigma);
y_cens ~ weibull_ccdf(alpha,sigma);

with two different variables, y_obs for the ordinary observations and y_cens for right censored. If we need to do left censoring, it's just the CDF instead of the CCDF. If we need to do both, we need a new notation; see #1154