deprecate the "target an aggregate with its key" feature

[gdementen]

Reference: #983 (comment)

Since I am unsure whether it is used by users, we need to deprecate it first.

Priority high, so that we can remove it and all the cruft that goes with it.

[gdementen]

also, it is even harmful in some cases (in the demo model there are a few occurrences where users need to use:

group >> None to work around this unwanted feature