refactor task hierarchy and introduce unsupervised and cluster task
start cluster learners
fix a few minor things for clustering
fix XMeans tests and add support for clustering to simple tests
add DB index for clustering performance evaluation
fix cluster performance measure
suggest clusterSim for clustering performance measure
ref package help for doc of clustering measure
base test for clusterers and some small related fixes
support plotting for clustering
update doc and NAMESPACE
fix up roxygen
complete parameters for XMeans
add dbscan
simple k means
add EM
more clustering measures
Merge branch 'clustering' of github.com:berndbischl/mlr into clustering