export getRequiredParams

[berndbischl]

@ja-thomas
do we still need this?

[coveralls]

Coverage remained the same at 96.005% when pulling df41802 on exportReqParNames into 82b8149 on master.

[ja-thomas]

We wanted to use that in mlrMBO, but I think it's not necessary anymore

[berndbischl]

ok i looked at this.
i still think this is useful and not much effort.

can you pls doc and (extend) test?
also it is probably better to have "getRequiredParams" and return a parset?
(as you can easily get the ids from that)

[berndbischl]

looks like we have this in PR #159?