Tutorial: some changes

[gdementen]

• rename pop variable to population everywhere
• in relabelling section:
  • move set_labels(dict) before set_labels(list) and set_labels(func), because this the simpler, most useful version.
  • add a big fat warning about set_labels(list) in both the tutorial and api doc because this is a big source of mistake

[alixdamman]

• show how to loop over an axis (over axes)

In arithmetic operations:

• remove matrix multiply
• Add more (many) examples of broadcasting.
• Replace examples with aggregation matrices by something else.

[alixdamman]

• rename section "Load And Dump Arrays, Sessions, Axes And Groups" as "Load And Dump Arrays"
• move subsection "Loading and Dumping Sessions" to "Working With Sessions"

[gdementen]

• add an example of .clip with arrays as minval and maxval (at least in API doc, and possibly in tutorial too)

[alixdamman]

• split the "Transforming Arrays" section into "Manipulating Axes And Labels of Arrays", "Combining Arrays" and "Aligning Arrays" sections.
• split the "Transforming Arrays" section into "Manipulating Axes And Labels of Arrays" and "Combining Arrays" sections.
• move the "Aggregations" part to a separated notebook

[alixdamman]

• Add more examples of how to use the plot() method

[gdementen]

* split the "Transforming Arrays" section into "Manipulating Axes And Labels of Arrays", "Combining Arrays" and "Aligning Arrays" sections.

Aligning Arrays is not a very intuitive section title. I am unsure what's better, but I doubt users would know what it means.