cbook alltrue() and onetrue()

However, it looks like onetrue() is never used and

    > alltrue() is used in lines.py and backend_ps.py, but is
    > imported from numerix not cbook - so can these cookbook
    > functions be deleted?

Since we have no way of knowing if code outside matplotlib is using
these, it seems like the path of least resistance is simply to fix the
bug.

JDH