Pivot rendering in the Koltin notebook plugin is slow

[ermolenkodev]

While working with a relatively big dataset (~10m rows), I observed that rendering the table for a pivot result is extremely slow—about 10 seconds for a 10m rows dataframe.
After a brief investigation, I found out that the problem is with converting the pivot object to a dataframe. That is implemented like that pivot.frames().toDataFrame()
TODO: investigate what is the bottleneck and optimize the Pivot->Dataframe conversion process