Harmonize labels names and use centering

A few reformating as well, commit mainly about consistency.

book/history/sourdough-history.tex

@@ -26,7 +26,7 @@ waters, another single-celled life form, \emph{archaea}, also thrived. These
 organisms inhabit extreme environments, from boiling vents to icy waters.
 
 \begin{figure}[!htb]
-\centering
+  \centering
   \input{figures/fig-life-planet-sourdough-timeline.tex}
   \caption[Sourdough microbiology timeline]{Timeline of significant events
     starting from the first day of Earth's existence,
@@ -69,7 +69,7 @@ Ancient Jordan~\cite{jordan+bread}. Looking at the earth's timeline sourdough
 bread can be considered a very recent invention.
 
 \begin{figure}[!htb]
-\centering
+  \centering
   \input{figures/fig-sourdough-history-timeline.tex}
   \caption[Sourdough history timeline]{Timeline of significant discoveries and
   events leading to modern sourdough bread.}%
@@ -81,10 +81,11 @@ bread are, however, unknown. One of the most ancient preserved
 sourdough breads has been excavated in Switzerland~\cite{switzerland+bread}.
 
 \begin{figure}[ht]
+  \centering
   \includegraphics[width=\textwidth]{einkorn-crumb}
   \caption[Ancient Einkorn flatbread]{An ancient Einkorn flatbread. Note the
       dense crumb structure.}%
-  \label{einkorn-crumb}
+  \label{fig:einkorn-crumb}
 \end{figure}
 
 Another popular story is that a lady in Egypt was making
@@ -193,9 +194,10 @@ mill marked a significant advancement in industrial technology for bread
 making~\cite{evans+mill}.
 
 \begin{figure}[ht]
+  \centering
   \includegraphics[width=\textwidth]{sourdough-stove}
   \caption{A bread made over the stove without an oven.}%
-  \label{sourdough-stove}
+  \label{fig:sourdough-stove}
 \end{figure}
 
 The biggest advancement of industrial breadmaking happened in \num{1857}.