added first implementation of mean-variance tree



git-svn-id: svn://svn.r-forge.r-project.org/svnroot/partykit@513 255f4249-2263-4417-b1ea-8498779e27a7
touch-ups in fitted and added examples for parameter extraction



git-svn-id: svn://svn.r-forge.r-project.org/svnroot/partykit@514 255f4249-2263-4417-b1ea-8498779e27a7