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/sourdough-starter/sourdough-starter.tex

@@ -127,9 +127,11 @@ inverted one on top. The container shouldn't be airtight,
 you still want some gas exchange to be possible.
 
 \begin{figure}[!htb]
-  \includegraphics{figures/fig-starter-process.pdf}
+\begin{center}
+  \input{figures/fig-starter-process.tex}
   \caption{The process of making a sourdough starter from scratch.}%
   \label{fig:sourdough-starter-process}
+\end{center}
 \end{figure}
 
 Now an epic battle begins. In one study scientists
@@ -248,11 +250,13 @@ yeast has been isolated like this from century old sourdough
 starters.
 
 \begin{figure}[!htb]
-  \includegraphics{figures/fig-starter-readiness.pdf}
+\begin{center}
+  \input{figures/fig-starter-readiness.tex}
   \caption{A flow chart showing you how to determine if your sourdough starter is ready to be used.
   For checking readiness look at a size increase and take note of your starter's smell. Both are important
   indicators to check for readiness.}%
   \label{fig:sourdough-starter-readiness}
+\end{center}
 \end{figure}
 
 The key signs to look at are bubbles that you see in your starter
@@ -315,13 +319,15 @@ yeast part of your sourdough and balance the fermentation.
 \section{Maintenance}
 
 \begin{figure}[!htb]
-  \includegraphics{figures/fig-starter-maintenance.pdf}
+\begin{center}
+  \input{figures/fig-starter-maintenance.tex}
   \caption{A full flowchart showing you how to conduct proper sourdough starter maintenance. You can use a
   piece of your dough as the next starter. You can also use left-over starter and feed it again. Choose an
   option that works best for your own schedule. The chart assumes that you are using a starter at a 100 percent
   hydration level. Adjust the water content accordingly when you use a stiff
 starter.}%
   \label{fig:sourdough-maintenance-process}
+\end{center}
 \end{figure}
 
 You have made your sourdough starter and your first bread. How do you perform