draw_likert frustrations

[acoppock]

A common use case for me is drawing likert POs on the basis of a continuous latent variable.

right now, draw_likert returns a factor, so it's clunky to put on the LHS of an estimator. I always have to as.numeric it.

Also, draw_likert doesn't appear to support an arbitrary number of breaks.

While we're at it, we should also update the documentation. I vote for separate man pages for all of the draw_* functions, as right now it's very hard to figure out which function to use.

[graemeblair]

+1 to separate man pages; very confusing as is