Potential to use .frequencies() method in /tutorial.html

[hyanwong]

At the moment the tutorial.html page says:

"At time of writing, we don’t have a built-in allele_frequency method, so we’ll use the following snippet"

But now we have Variant.frequencies():

import msprime
ts = msprime.sim_mutations(msprime.sim_ancestry(10, sequence_length=10, random_seed=1), rate=1, random_seed=1)
for v in ts.variants(samples=np.arange(5)):
    print(v.frequencies())

I didn't fully look at the example, but would this make the whole thing easier, @petrelharp ?