keys should be stored in ParamSet

[jakob-r]

We call makeParamSet(..., keys = c("p", "q")) but we do not store the keys in the parameter set.
They are just used for some kind of feasibility check - which unfortunately can not be done later again because the keys are missing. We can use all.vars later but then we could directly do this and not force the user to pass keys.

[berndbischl]

i guess this is correct

[jakob-r]

What is the preferred option? drop keys or make the argument optional. In case they are not passed us all.vars on the expressions to obtain the keys?
Save them in a slot or not?