mb706

What should be on it? * CPO lifecycle * CPO information getters * listing of CPOs * important meta-CPOs (cpoCase, cpoCbind, ...) * makeCPO on a separate page

Otherwise problems related to the one in mlr-org/mlr#2472 could appear.

Differentiate (and warn about) objects with NULLCPO retrafo vs. objects w/o any retrafo

1

E.g. ```R x = data %>>% cpo() inverter(x) # may be NULLCPO, no warnings or anything about that. inverter(data) # should also warn that a CPO was never assigned inverter(data,...

`!cpoCollapseFact` should show one "`cpo.train`" and one "`cpo.retrafo`", instead it shows one "`cpo.trafo`" and one "`cpo.retrafo`"

If [this line](https://github.com/mlr-org/mlrCPO/blob/50ce83712c8ff98a2fa3b1b16bfc00e0d1e4ad2d/R/CPO_meta.R#L377) fails, the cache entry still contains the previous value; the wrapped CPO should be called before `cache.index` is updated.

`cpoFilterFeatures(method = x)` currently does not work if `x` was created by the user using `makeFilter`.

E.g. `!cpoDropConstants` has a `cpo.trafo`, even though the function is given as `cpo.train`.