Improve AddDsl

[AndreiKingsley]

Now API is very confusing.
Proposal:

df.add { df ->
    col("year of birth") { 2021 - age }
    col(df.age > 18 named "isAdult")
    colGroup("details") {
        col(df.name.lastName.length() named "last name length")
        col("full name") { name.firstName + " " + name.lastName }
    }
    col(df.colA)
}