Add examples of using numpy traversal methods

[benjeffery]

See tskit-dev/tskit#1788

[hyanwong]

I discussed this with @jeromekelleher - we wondered whether the example in the tutorial would be better moved to a section of the docs (not the tutorials), designed for simple examples like this. There we could illustrate 4 methods of getting the total branch length of a tree:

1. Using tree.nodes() to iterate through
2. Using numpy arrays
3. High performance, using numpy arrays and numba
4. Using the built-in branch length method

It would be nice to illustrate the difference in speed between the two, by picking an example that took (e.g.) 1 sec for method (1).

[hyanwong]

An example of 2. is given in tskit-dev/tskit#1788 (comment)