refactored the documentation as well as the code of TreeGrammarDecomposition

git-svn-id: http://gapt.googlecode.com/svn/trunk/source@2100 85d21ef8-add0-11de-ae77-5bfb9c5a3f34
fixed a bug by reverting resulting in wrong grammars
fixed an issue with languages, where we have to deal with non-linear term growing

git-svn-id: http://gapt.googlecode.com/svn/trunk/source@2101 85d21ef8-add0-11de-ae77-5bfb9c5a3f34