Rework viz tutorial

As pointed out by Ben Haller, the code to generate the example tree sequence in the viz tutorial is completely hidden. This revamps the tutorial so that the instructive part of the text uses hidden tree sequences, but the examples at the end (which now include the 3D example) are self-contained, and so can be easilty executed by copy-and-paste.l

This relies on a new tskit version being released with tskit-dev/tskit#2650 and tskit-dev/tskit#2646 merged

Author: hyanwong

data/viz_7_trees_of_4_tips.trees

@@ -7,7 +7,7 @@ scikit-allel
 tqdm
 numpy
 networkx
-tskit>=0.5.1
+tskit>=0.5.4
 msprime>=1.0
 stdpopsim
 tskit-book-theme==0.3.2