Duplicate Column accessors functions

[Jolanrensen]

Especially in the Columns Selection DSL the library allows the user to create column accessors using either the constructors column(), columnGroup(), etc. from constructors.kt as well as col(), colGroup(), ""<>(), ""[""], etc. (now with runtime checks) from the selection DSL ColumnSelectionDsl.kt, col.kt, and colGroup.kt.

Having both is confusing. We should keep one of the two variants, or have them be exactly the same. Column shortcuts like ""<>() should be available library-wide too I think, as it's common enough.

Can be handled after https://github.com/Kotlin/dataframe/pull/372