Fix chemical formula display (#65)

This uses the chemformula package to improve the display of chemical
formulas.

book/bread-types/bread-types.tex

@@ -249,7 +249,7 @@ state of my doughs.
   \includegraphics[width=1.0\textwidth]{injera-pancake-texture.jpg}
   \centering
   \caption{A sourdough pancake made with teff flour. The pockets are coming from
-  the evaporated water and CO2 created by the microbes.
+  the evaporated water and \ch{CO2} created by the microbes.
   The image has been provided by Lukasz Nowak via Wikipedia}
 \end{figure}
 
@@ -284,7 +284,7 @@ option opt for around 0.1-0.5cm depending on what you like.
   \includegraphics[width=1.0\textwidth]{einkorn-crumb.jpg}
   \centering
   \caption{The crumb of a flat bread made with einkorn as flour. Einkorn
-  is very low in gluten and thus does not trap as much CO2 as a wheat based
+  is very low in gluten and thus does not trap as much \ch{CO2} as a wheat based
   dough. To make the dough fluffier use more water or consider adding
   more wheat to the mix of your dough.}
 \end{figure}