Use input figs (#123)

* Make figs includable in main document

Remove the capability to build them as standalone document but we can
include them in the main document.  It should simplify things down the
road.

* Replace tikx pics

* Remove figures compilation from makefile

No need to compile figs to pdf anymore, at least to build the books

* Delete svg figures from ebook build

* Create png for TikZ figures

    - Add export_figures back
    - Build a pdf from the the TikZ in standalone mode
    - Change the cleanup to deal with those changes
    - Remove trailing spaces..

* Replace centering by an environment

More LaTeX idomatic

* Increase clean_figures robustness

as we use temporary tex files (.tex.in) we can't run clean_figures twice in a
row

* Center TikZ figures

Because it looks better

* Remove png building

Seems to struggle on CI with \\\b or something, we don't absolutely need
it right now so let's get rid of it.

* Remove trgt_figures dependency for pdf and ebooks

This should not be needed now that we include the TikZ directly, only
needed for png generation.

* Revert "Remove png building"

This reverts commit fdd542de57.

This is now fine to add the code back, so that it can be debugged when
times come.

book/wheat-sourdough/wheat-sourdough.tex

@@ -43,9 +43,11 @@ that tastes much better than any store-bought bread.
 \section{The process}
 
 \begin{figure}[!htb]
-  \includegraphics{figures/fig-wheat-sourdough-process.pdf}
+\begin{center}
+  \input{figures/fig-wheat-sourdough-process.tex}
   \caption{The typical process of making a wheat-based sourdough bread.}%
   \label{fig:wheat-sourdough-process}
+\end{center}
 \end{figure}
 
 The whole process of making great sourdough bread starts with
@@ -95,13 +97,15 @@ doesn't have a good balance of yeast to bacteria, so will your
 main dough.
 
 \begin{figure}[!htb]
-  \includegraphics{figures/fig-wheat-sourdough-starter-process.pdf}
+\begin{center}
+  \input{figures/fig-wheat-sourdough-starter-process.tex}
   \caption{The process to check your sourdough starter when making wheat-based doughs. In practice
   I~frequently use a stiff sourdough starter. The stiff starter features enhanced yeast activity. In that case, you can
   use the same ratios as shown in the chart except for the water quantity. The stiff starter has a hydration of 50 to
   60 percent. So you would have half the shown water quantities, i.e., if the chart shows 100 g of water, use 50 to 60 g of water
   for your stiff starter.}%
   \label{fig:process-starter-wheat-sourdough}
+\end{center}
 \end{figure}
 
 Generally, think of the dough you are mixing as a big starter with salt.
@@ -544,9 +548,11 @@ all the gases during the fermentation process. Without the gluten network,
 the gases would just diffuse out of your dough.
 
 \begin{figure}[!htb]
-  \includegraphics{figures/fig-kneading-process.pdf}
+\begin{center}
+  \input{figures/fig-kneading-process.tex}
   \caption{The gluten development process for a wheat-based dough.}%
   \label{fig:wheat-sourdough-kneading-process}
+\end{center}
 \end{figure}
 
 It might sound odd, but the most important part of kneading is waiting. By
@@ -751,12 +757,14 @@ is much larger. The doughs are perfect to be made in a
 machine.
 
 \begin{figure}[!htb]
-  \includegraphics{figures/fig-bulk-fermentation.pdf}
+\begin{center}
+  \input{figures/fig-bulk-fermentation.tex}
   \caption{During the bulk fermentation, multiple doughs are fermented together in bulk.
   A challenging aspect of homemade sourdough bread is to determine when this stage of
   fermentation is completed. This chart shows multiple available options to check on the bulk
   fermentation progress.}%
   \label{fig:bulk-fermentation}
+\end{center}
 \end{figure}
 
 Experienced bakers will tell you to go by the look and feel of
@@ -1086,10 +1094,12 @@ The step is required if you are making multiple loaves in one
 batch. It is optional if you are making a single loaf.
 
 \begin{figure}[!htb]
-  \includegraphics{figures/fig-dividing-preshaping.pdf}
+\begin{center}
+  \input{figures/fig-dividing-preshaping.tex}
   \caption{Dividing is only required when you are making multiple loaves in a
   single dough batch.}%
   \label{fig:dividing-decision-tree}
+\end{center}
 \end{figure}
 
 The goal of dividing your dough into smaller pieces is to portion
@@ -1198,10 +1208,12 @@ your environment.
 \section{Shaping}
 
 \begin{figure}[!htb]
-  \includegraphics{figures/fig-shaping-process.pdf}
+\begin{center}
+  \input{figures/fig-shaping-process.tex}
   \caption{A schematic visualization of the shaping process including checks
   for an overfermented dough.}%
   \label{fig:shaping-decision-tree}
+\end{center}
 \end{figure}
 
 Shaping will give your dough the final shape before baking. After
@@ -1418,10 +1430,12 @@ their biochemical processes. More research is needed on the topic
 of retarding and flavor development.
 
 \begin{figure}[!htb]
-  \includegraphics{figures/fig-proofing-process.pdf}
+\begin{center}
+  \input{figures/fig-proofing-process.tex}
   \caption{A schematic overview of the different steps of the sourdough proofing process. The proofing technique to choose
   depends on your availability and schedule.}%
   \label{fig:proofing-process}
+\end{center}
 \end{figure}
 
 To me, the sole purpose of cold proofing is its ability to allow you