Reach the feature parity between converting columns and adding columns operations

[koperagen]

The problem: no feature parity between converting columns and adding columns

We have toDataFrame for converting list to DataFrame reflectively. unfold for converting column to ColumnGroup reflectively
I think we lack some logical symmetry when it comes to doing same what unfold does, but ADDING a column instead. If we expect that it's sometimes desirable to convert Column->ColumnGroup according to T properties, seems natural that add should allow the same

Possible options:

df.add {
    "" from properties { }
    "" from unfold { }
    unfold { } into "abc"
 }