Add an example using `PineAPPL` to compute predictions

Created by: scarlehoff

Added a notebook to the examples folder where validphys is monkeypatched to utilize pineappl grids instead of fktables.

The code for this will not be integrated in validphys (validphys will keep using only fktables because changing to grids can open many cans of worms that I don't want to open) but as you can see in the notebook the code is quite simple.

This deals (in a way) with parts of https://github.com/NNPDF/nnpdf/issues/1961

ps: If there is any function in validphys for which the monkey patching in the notebook doesn't produce the desired result it might be due to said function using directly convolute instead of results. Please open an issue / PR because the more we make the validphys API be consistent the easiest it becomes to do things like this in a simple manner!