Move/Insert after nested column breaks

[Jolanrensen]

To reproduce:

val df = dataFrameOf("a", "b", "c")(1, 2, 3, 4, 5, 6)
val grouped = df.group { a and b and c }.into("g")

grouped.insert("d") { g.b  * g.c }.after { g.b }
// or
grouped.move { g.a }.after { g.b }

The problem lies with move, as insert calls move as part of its implementation.

We should either prohibit this behavior with a helpful exception or fix the behavior.