Fix strahler.swc
Add a first point at the root node in order not to start with a bifurcation.
Because starting with a bifurcation in SWC is broken (but fixed in morphio)
# https://commons.wikimedia.org/wiki/File:Flussordnung_(Strahler).svg
1 1 0 0 0 1 -1
- 2 3 0 2 0 1 1
- 3 3 -1 2 0 1 2
- 4 3 0 3 0 1 2
- 5 3 10 3 0 1 4
- 6 3 10 2 0 1 5
- 7 3 9 2 0 1 6
- 8 3 11 2 0 1 6
- 9 3 10 4 0 1 5
-10 3 9 4 0 1 9
-11 3 11 4 0 1 9
-12 3 0 4 0 1 4
-13 3 -1 4 0 1 12
-14 3 0 5 0 1 12
-15 3 -5 5 0 1 14
-16 3 -5 6 0 1 15
-17 3 -5 4 0 1 15
-18 3 5 5 0 1 14
-19 3 5 6 0 1 18
-20 3 5 4 0 1 18
+ 2 3 0 0 0 1 1
+ 3 3 0 2 0 1 2
+ 4 3 -1 2 0 1 3
+ 5 3 0 3 0 1 3
+ 6 3 10 3 0 1 5
+ 7 3 10 2 0 1 6
+ 8 3 9 2 0 1 7
+ 9 3 11 2 0 1 7
+10 3 10 4 0 1 6
+11 3 9 4 0 1 10
+12 3 11 4 0 1 10
+13 3 0 4 0 1 5
+14 3 -1 4 0 1 13
+15 3 0 5 0 1 13
+16 3 -5 5 0 1 15
+17 3 -5 6 0 1 16
+18 3 -5 4 0 1 16
+19 3 5 5 0 1 15
+20 3 5 6 0 1 19
+21 3 5 4 0 1 19
