Use Siunitx package for dealing with units (#129)

It is complicated :
[1] The International System of Units (si), https://www.bipm.org/en/
measurement-units/.
[2] International System of Units from nist, http://physics.nist.gov/cuu/Units/
index.html.

And one will never get it right (space or not, half-space?) nor
consistent so using that instead.

I am not sure how times and hours, when to write digits and when in
letter so I did not change much..

Did not touch the tables as ebooks on github actions seems to break when
you look at them funny.

Co-authored-by: Cedric <ced@awase.ostal>

book/figures/fig-wheat-sourdough-starter-process.tex

@@ -1,13 +1,13 @@
 \begin{tikzpicture}[node distance = 3cm, auto]
   \node [decision] (init) {\footnotesize Starter last fed within 3 days?};
   \node [block, right of=init, node distance=4cm] (feed_no_branch)
-    {\footnotesize Feed starter twice. 48 hours before and 6--12 hours before};
+      {\footnotesize Feed starter twice. \qty{48}{\hour} before and \qtyrange{6}{12}{\hour} before};
   \node [block, below of=feed_no_branch, node distance=3cm] (feed_yes_branch)
-    {\footnotesize Feed starter once 6--12 hours before making dough};
+      {\footnotesize Feed starter once \qtyrange{6}{12}{\hour} before making dough};
   \node [block, right of=feed_no_branch, node distance=6cm] (high_ratio)
-    {\footnotesize Use a 1:10:10 ratio. 10g starter, 100g flour, 100g water};
+    {\footnotesize Use a 1:10:10 ratio. \qty{10}{\gram} starter, \qty{100}{\gram} flour, \qty{100}{\gram} water};
   \node [block, right of=feed_yes_branch, node distance=3cm] (low_ratio)
-    {\footnotesize Use a 1:5:5 ratio. 10g starter, 50g flour, 50g water};
+    {\footnotesize Use a 1:5:5 ratio. \qty{10}{\gram} starter, \qty{50}{\gram} flour, \qty{50}{\gram} water};
   \node [block, below of=high_ratio, node distance=6cm] (check_starter)
     {\footnotesize Check if starter is ready to be used};
   \node [decision, below of=init, node distance=6cm] (size_check)