TreeGrammarDecomposition: updated javadoc, refactored abstract class to be more obliging in the general case when providing  different MinCostSAT formulations than QMaxSAT

git-svn-id: http://gapt.googlecode.com/svn/trunk/source@2110 85d21ef8-add0-11de-ae77-5bfb9c5a3f34