added mtry option for mob

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/partykit@516 255f4249-2263-4417-b1ea-8498779e27a7
first steps towards mixforest

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/partykit@517 255f4249-2263-4417-b1ea-8498779e27a7