Resolve "Move `a4paper` option to class options"

Closes #9 (closed)

I would guess that geometry uses the default paper format of the installation, when nothing is specified. So for our local installations that would be a4paper, while in the US it is likely usletter instead. The new version is now passing paper=a4paper to geometry explicitly using the \geometry macro. This should still allow people to override the settings calling geometry in their main files, but at the same time ensure that arxiv uses A4 when that is not done.