Add rel_width argument for figsizes

[marvinpfoertner]

This PR adds the possibility to make smaller figures in full-width formats using a width parameter relative to the width of the page

[pnkraemer]

Thanks a lot for making this PR!

I left a few comments in the diff, mostly regarding a few things I would appreciate a little clarification on, but nothing major! :) Let me know if you would like to resolve this offline.