Tutorial - Getting Started: do no use axes objects defined in a previous section

[gdementen]

When looking at a subsection in isolation (e.g. Aggregation, if you are only interested in that), it is very confusing because the tutorial uses axes names without quotes and then users think it will work like that for their code too, but they do not see (or even if they see it, some do not understand the importance of this) that we defined variables for those axes.

We should probably use quotes for axes names all the way in the getting started, and show and explain the difference in a later section.