preparations for next cut-intro test run

git-svn-id: http://gapt.googlecode.com/svn/trunk/source@1816 85d21ef8-add0-11de-ae77-5bfb9c5a3f34
Made ExpansionTree feign being a subclass of ExpansionTreeWithMerges
(Implementation of basic datastructure tries to keep static type info about ETs not containing merges as far as possible)
Renamed ET Not to Neg
Implemented substitution on ExpansionTrees (merges are not applied yet)
Added unit tests for ET substitution


git-svn-id: http://gapt.googlecode.com/svn/trunk/source@1817 85d21ef8-add0-11de-ae77-5bfb9c5a3f34