Added a section to handle the case where the problem has not yet been
assigned to a user. This might occur when the problem template is
called form the problem editor before the sets have been assigned
to students. The problem is obtained as a global problem
then converted to a user_problem with default values. The problem
is not saved. This code should be reviewed to make sure there are no
hidden behaviors. Perhaps a group reading is in order once things
settle down. This squashes bug #34.