Fix maps in GEMs lab

[rasools]

Stefan:
In the GEM lab '03-Pathway-visualization', you can display the map for the E. coli central carbon metabolism by using the following code in cell 3:
b=escher.Builder('e_coli_core.Core metabolism')
escher.Builder.save_html(b, filepath='data/central_carbon_e_coli.html')
The map will then show up as html in your data folder (which you can access from the location where you open the notebook)

This applies to all cells in the subsequent labs where you are trying to visualize maps with escher.Builder(). Assign the builder to a variable (as in e.g b=escher.Builder('e_coli_core.Core metabolism')) and then save it to html (as in escher.Builder.save_html(b, filepath='data/central_carbon_e_coli.html'))