Improve the legibility of the code for map inversion

[TheoVerhelst]

I updated the notebook on map inversion to make the map extract and inversion a little bit easier to read and use.

[bluescarni]

Thanks @TheoVerhelst! It looks good to me, though I would also ask @darioizzo to take a quick look if he can before I merge.

[darioizzo]

@bluescarni Good for me too, @TheoVerhelst thansks for improving the logic ... eventually this notebook will need anyway an update as we are now talking and publishing about Event Transition Tensors.