Add option to have plots in dimensions

[mulvrova]

By default all plots are in non-dimensional space. An option to rescale variables back to dimensions should be available.

[amorison]

This is done with commits 59376f4 and 2dd52bd. The approach implemented here is rather crude and not very flexible. That should be fine for now, but maybe it would be worth using an external library such as Pint to gain flexibility.