DataFrame ans Kandy need to use dev versions to work (together) in notebooks

[Jolanrensen]

There are two problems with the latest DataFrame and Kandy version.

• Due to Notebooks fail with T : Comparable<T & Any>? notation dataframe#1116 1.0.0-Beta2 causes issues in notebooks. This is solved by switching to 1.0.0-dev-7089.
• Kandy 0.8 and DataFrame 1.0.0-x can clash with generated column accessors. This is solved by kandy 0.8.1-dev-67, kandy stats 0.4.2-dev-2.

I merged #505 so that these issues won't occur, but the downside is that they are developer versions.

If the relevant issue is fixed and Kandy is updated, this needs to be moved to "normal" versions again.