Add support for explode map

[asm0dey]

Currently, it looks like that explode on Maps won't work. Kindly add support

[OlivierBlanvillain]

Is this supported by Spark? If so, it should only be a mater of adding an implicit for Map alongside the other ones:

frameless/dataset/src/main/scala/frameless/functions/UnaryFunctions.scala

Lines 70 to 77 in c763176

	trait CatalystExplodableCollection[V[_]]
	object CatalystExplodableCollection {
	implicit def explodableVector: CatalystExplodableCollection[Vector] = new CatalystExplodableCollection[Vector] {}
	implicit def explodableArray: CatalystExplodableCollection[Array] = new CatalystExplodableCollection[Array] {}
	implicit def explodableList: CatalystExplodableCollection[List] = new CatalystExplodableCollection[List] {}
	implicit def explodableSeq: CatalystExplodableCollection[Seq] = new CatalystExplodableCollection[Seq] {}
	}

[asm0dey]

Of course it does. It produces 2 columns: key and value (comparing to existing implementation)

[OlivierBlanvillain]

Cool :) If you want to give this a try it should be fairly straightforward first contribution!

[asm0dey]

I'm absolutely have no idea on how to implement this: level of knowledge about spark is too low

[ayoub-benali]

@OlivierBlanvillain CatalystExplodableCollection has the same issue as CatalystSizableCollection
It has only one hole for the type while Map needs two.
The easiest fix I see is to implement a second explode that accepts a Map instead of a CatalystExplodableCollection. Just like I did here #255

If that works for you I can make a PR