Set up CI and run nbval on notebooks

[fangohr]

Purpose: check that notebooks on binder will execute correctly.

In principle this should happen automatically if we always sync from oscovida/tools/pelican/content/ipynb, but

(i) this is a manual process and we may forget and

(ii) we also need to make sure the notebooks in here work with the requirements.txt we define for this binder environment.

Example command:

cd ipynb && py.test --nbval-lax 20*ipynb 14*ipynb tu*ipynb 
cd ipynb && Germany.ipynb US-California.ipynb Hungary-Baranya.ipynb Germany-Hamburg-SK-Hamburg.ipynb 

(We don't want to run all of the notebooks for all regions as this would take too much time.)

[fangohr]

Addition: also the syncing of the 'normal' notebooks (such as German.ipynb) doesn't happen automatically, so we need to check they work as well. (At the moment, they don't ;-)).