fix: Python 3 compatibility in examples

- speed_highd.py: Cast float to int for evolve() argument
- genealogies_with_selection.py: Use integer division (//) instead of
  float division (/) for locus index

In Python 3, `/` always returns float, while `//` returns int.
Functions expecting integer arguments now get the correct type.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: corneliusroemer

examples/genealogies_with_selection.py

@@ -38,8 +38,8 @@
 #set the effect sizes of the mutations that are injected (the same at each site in this case)
 pop.set_fitness_additive(np.ones(L)*s)
 
-#track the genealogy at a central locus L/2 (which one doesn't matter in the asexual case)
-pop.track_locus_genealogy([L/2])
+#track the genealogy at a central locus L//2 (which one doesn't matter in the asexual case)
+pop.track_locus_genealogy([L//2])
 
 #initialize the populations
 pop.set_wildtype(pop.carrying_capacity)

examples/speed_highd.py

@@ -41,7 +41,7 @@
 
         pop.set_wildtype(N)                     # set a wildtype population of size N
 
-        pop.evolve(1.0 / (L * (mu + r)))        # evolve until equilibrium
+        pop.evolve(int(1.0 / (L * (mu + r))))   # evolve until equilibrium
 
         # run for G generations to measure execution time
         t1=time.time()