Fixed expansion tree extraction of proofs containing weakenings
When necessary, top/bottom are used as quantifier instances for the et of the formula introduced by weakening (top for the succedent, bottom for the antecedent)
Fixes #261


git-svn-id: http://gapt.googlecode.com/svn/trunk/source@1846 85d21ef8-add0-11de-ae77-5bfb9c5a3f34