Missing guide for connecting unsupported dataframe-like libraries to DataFrame

[Jolanrensen]

Because DataFrame is such a lightweight and close-to-stdlib library, it's fairly trivial to connect other libraries and convert their structures to a dataframe and back. Still, many people don't know that this is possible and may expect some difficult type-mapping to be required, while this often isn't the case.

I believe we should provide a little guide with some examples for how to do this. For instance, we could provide an example for

• Apache Spark
  • with kotlin spark api https://github.com/Kotlin/kotlin-spark-api/wiki/Kotlin-DataFrame-interoperability
  • without kotlin spark api
• Exposed
  • https://datalore.jetbrains.com/report/static/JcoOsBmzVtPjSsAd1bU6Oh/VHtZbM5LWZ1NoOsO00szgF
• Generic list of data classes?

If needed, we could provide type mappings to generate data schemas too, but often that's not even necessary.

Of course, these examples could turn into a fully fletched io source if needed, but providing them as example could prove valuable I think :)