Add a points file for no-answer responses to questions.

[mwest1066]

At the moment we can award points for any answer to a question by adjusting the points file, but we can't award points for a question to students who don't answer it at all. For invalid questions with no answer, some students might just not fill in anything.

We should have a P_na.csv file generated by proc-lib that is initially all zeros, but which will be read by proc-ans and used to give points for non-answers.