update API so that functions expect event_oberved array instead of censorship array

This is a big one!

I tried to have a look around each change and see if any other lines needed updating, but I may have missed some. Tests are passing, though!

(test_aalen_additive_plot and test_aalen_additive_smoothed_plot seem to be failing sometimes but not others, for no apparent reason. Not sure what's going on there)