Add an example that illustrates set_on_model for Optimize

Here is a bare bones example:

from z3 import *
import sys

opt = Optimize()

def on_model(m):
    print("model", m)
    sys.stdout.flush()
    
opt.set_on_model(on_model)

#opt.set("maxsat_engine","wmax")
opt.set("enable_lns", True)
opt.from_file("../smtlib.txt")

set_option(verbose=2)

opt.set(timeout=3000)
opt.check()
print(opt.model())