Add alt text to figures in reports

[Bisaloo]

This is especially useful for visitors using screen readers.

It may also be a good moment to switch to knitr's new syntax for chunks. Another nice side effect is that diffs in git will be clearer.

I need to ask what is the best practice to have a meaningful alt text in the case of continuously updated figures. We cannot really describe what is going on since we don't know it yet.