bug fix

tutor responses removed (default to [level 1])
bug when student column has only [...] fixed: will not generate submit
tree in this case.
Revert "bug fix"

This reverts commit 4a71d0d7d942e9a2c9430d645d662f72b9324f68.
Revert "Revert "bug fix""

This reverts commit 5cb12142672572f33c9058ae77c1a379b4ccf69a.