Improve visualize_pathways_levelX functions

See requested changes here: https://github.com/inbo/alien-species-portal/issues/186

It should be possible to also plot pathways that are not in the data (i.e. pathways with count 0). 
In order to allow this, we should be able to fill the pathways parameter with values that are not in the data, and to make sure that each element in the pathways parameter gets a (possible empty) bar.