Fix of reported bug (by Hannah :) with NAs in rstree()/pctree().

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/prefmod@206 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Added R/pctree-psychotools.R with several copies of unexported functions from package psychotools. Adjusted function calls accordingly.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/prefmod@207 edb9625f-4e0d-4859-8d74-9fd3b1da38cb