make single pipeop application easier

[mb706]

p = po("encode")
task_preproc = p$train(list(task))[[1]]

[mb706]

potential problem: we don't want to tempt people into doing the wrong kind of preprocessing on train and test set

[mb706]

suggestion (bb):

outdata = preproc("encode", indata, paramlist, state = state)

[mb706]

maybe we want to have preproc(po("encode", paramlist)) instead, more consistent and would also work with PipeOps that are not in po().